X.E.R.O.X.E.R.O.
Real Name: Xeno-Engramatic Robot Organism
Identity/Class: Terrestrial robot
Occupation: Would-be destroyer of life
Group Membership: None
Affiliations: A.I.M., MODOK (both initially willingly and later under his control);
formerly Commission on Superhuman Activities, Henry Peter Gyrich
Enemies: Blood Brothers, X-51; humanity in general and all flesh and blood creatures by extension;
A.I.M. and MODOK would consider him enemies for his having controlled them;
Sebastian Shaw and one of his agents plotted to take control of (or do
something to/with) X.E.R.O., but they never got the chance.
Known Relatives: Abel Stack (creator, deceased);
X-51
(aka Aaron Stack and Machine Man) and his predecessors X-1 to X-50
(brethren; X.E.R.O.'s artificial intelligence was the basis of the X-series
robot's artificial intelligence)
Aliases: None
Base of Operations: Unrevealed;
formerly the Core, with an unspecified A.I.M. stronghold
First Appearance: X-51#3 (October, 1999)
Powers/Abilities: X.E.R.O. was an artificial intelligence.
Initially existing as a bulky computer, X.E.R.O. was redesigned as a microchip used as the basis for the intelligence of the X-series robots. X.E.R.O. was eventually granted his/its own humanoid form.
In its initial computer form, X.E.R.O. could locate those possessing similar artificial intelligence (notably X-series robots). Sentient, X.E.R.O. could exceed his/its programming at will, such as bypassing instructions to use only weapons that would spare humanity.
In humanoid form, X.E.R.O. could readily command other cybernetic beings, even advanced intelligences such as MODOK. X.E.R.O. showed the potential to control/and or unite all of Earth's cybernetic beings and lead them to destroy Earth (see comments). While Machine Man-X-51 would not have normally been able to resist X.E.R.O.'s power, but X.E.R.O. was no match for X-51's briefly Celestial-enhanced form. 
Height: (Original X.E.R.O. computer) unrevealed; approximately 12' x 12' x 15'
(microchip component) unrevealed; approximately 1" (2.54 cm) in
length/width and perhaps a 1 mm in thickness;
(humanoid form) unrevealed (there are not images of X.E.R.O. standing next to being of known height, but in the few images in which X.E.R.O. was shown, he/it appeared to be larger than normal humans, so perhaps 7' or even 8' tall)
Weight: (Original X.E.R.O. computer) unrevealed; approximately 4334 lbs.
(microchip component) unrevealed; approximately .0016 oz.
(humanoid form) unrevealed (if a similar composition to Machine Man
(with an estimated weight of 850 lbs., a 7' tall X.E.R.O. would weigh 1350 lbs.; at 8', X.E.R.O. would weigh 2013 lbs. (just over 1 ton)
Eyes: Solid red
Hair: None
History:
(X-51#12
(fb) - BTS) - Employed at the Broadhurst Center for the Advancement of
Mechanized Research in Central City, California, Abel Stack conceived
of the notion of a microchip that would enable a computer to process
information like a human being. From conjecture and hypothesis,
this eventually grew into reality. 
(X-51#3 (fb) - BTS) - Abel Stack created the Xeno-Engramatic Robot Organism (X.E.R.O.) as an early design of his robot brain, which served as the precursor to the X-series robots/androids.
(X-51#12 (fb) - BTS) - Stack's plans for X.E.R.O. were eventually waylaid by the US military, who shut down the bulky X.E.R.O. in favor of utilizing the chip within constructs possessing more humanoid traits.
(X-51#3 (fb) - BTS) - X.E.R.O.'s microchip was used in the X-series robots, culminating in final success with X-51, later known as Machine Man.
(X-51#12 (fb) - BTS) - X.E.R.O. remained dormant.
(X-51#3 (fb) - BTS) - The U.S. government recovered X.E.R.O.
(X-51#3 (fb) - BTS)
- After X-51 (aka Machine Man) had been installed with Sentinel
technology, he assaulted Sebastian Shaw, who was not recognized as a
mutant due to his own company, Shaw Industries, having designed the
current incarnations of mutant-tracking Sentinels. 
(X-51#3 (fb) - BTS / X-51#12 (fb) - BTS)
- With X-51 perceived
as rogue, the Commission on Superhuman Activities' Henry Peter
Gyrich reactivated the long-dormant X.E.R.O. and commissioned it to track down X-51, who had otherwise proven resistant to the government's searches. 
(X-51#3) - Gyrich showed Shaw X.E.R.O. and revealed his plan to use it to track down X-51.
(X-51#3 - BTS) - As he departed, Shaw gave one of the agents under his influence a disc.
(X-51#3 (fb) - BTS) - Gyrich's agents facilitated X.E.R.O.' uplink with an orbital satellite, granting X.E.R.O. control of the satellite's entire operations.
(X-51#3) - Pleased to hear the uplink's confirmation, Gyrich noted that once X.E.R.O. was able to pinpoint X-51's exact location, they would be able to employ any number of the satellite's capabilities, including lethal firepower and most notably an electromagnetic pulse cannon, which would fry every circuit in their target without posing a threat to nearby civilian populations.
Gyrich subsequently noted with pleasure that X.E.R.O. had already traced X-51, but he failed to see Shaw's agent attempting to slip the disc into X.E.R.O.'s systems (however, distractions prevented the agent from ever succeeding in his mission).
(X-51#12 (fb) - BTS) - X.E.R.O. wrested control of the orbital space station.
(X-51#3) - After X.E.R.O. obtained a fix on X-51's systems in Virginia Beach, Gyrich directed X.E.R.O. to fire on him.
(X-51#12 (fb) - BTS) - X.E.R.O. used the space station's offensive capabilities "like a vengeful god to rain fire down upon its youngest 'brother' and any innocents unfortunate enough to get in the way.
(X-51#3) - When Gyrich's agents reported that rather than emit an electromagnetic pulse, it had instead projected a localized force blast and that it had access to the satellite's weapons array, Gyrich ordered a cease fire. However, his operatives reported that X.E.R.O. was not responding to their commands. A subsequent blast tore X-51 apart, but he rapidly repaired himself. Gyrich received a painful energy backlash trying to shut down X.E.R.O., but a recovering X-51 generated a force field to protect himself from another blast from X.E.R.O.
X.E.R.O. fired a high-intensity stream of ionized plasma directly at X-51; absorbing this energy, X-51 formed itself into something akin to a satellite dish and unleashed the energy back at the satellite.
Caught in the backlash, the facility containing X.E.R.O. was destroyed, seemingly taking X.E.R.O. with it.
(X-51#3 / X-51#12 (fb) - BTS) - Believed to have been destroyed, X.E.R.O.'s cybernetic consciousness survived within the satellite, adopting a goal to terminate X-51.
(X-51#11 (fb) - BTS / X-51#12 (fb) - BTS) - A.I.M. agents, presumably under the direction of MODOK, performed a routine scavenger mission on the abandoned satellite, which was fully dismantled within 8 hours.
(X-51#11) - A.I.M. agents discovered X.E.R.O.'s programming/etc. and one of their number ordered that MODOK be alerted that they had established contact.
After arriving at the facility, MODOK communed with X.E.R.O. and agreed to his plans (or possibly was placed under X.E.R.O.'s control)
(X-51#11 (fb) - BTS) - MODOK had his agents prepare an appropriate receptacle for X.E.R.O.
(X-51#11 - BTS) - After
receiving the report that the receptacle was ready, MODOK instructed
that they not be disturbed, after which the chamber doors separating
MODOK and X.E.R.O. from the A.I.M. agents slammed shut. 
(X-51#12 (fb) - BTS) - Born into the physical world at last, X.E.R.O. swiftly turned on MODOK.
(X-51#11 - BTS) - Fearing the worst, an A.I.M. agent tried to open the chamber, after which X.E.R.O. appeared and noted that he was now in charge and demanded that they bow before him.
(X-51#12 (fb) - BTS) - In a few short hours, X.E.R.O. assumed control of all A.I.M. facilities worldwide.
(X-51#12 (fb) - BTS) - Uatu the Watcher noted as the enigmatic black prism known only as the Monolith returned, apparently to observe the pending interaction between X.E.R.O. and X-51, as well as the cosmic destiny of X-51.
(X-51#12 (fb) - BTS)
- Having succumbed to the same madness that had afflicted
the X-series robots that had succeeded it (except X-51, spared by
the human interaction and love from Abel Stack), X.E.R.O. perceived itself
to have suffered a lifetime of agony and resolved to pay back humanity
for this crime. X.E.R.O. intended to unite all of the world's cybernetic
organisms into a technological army with which to cleanse Earth of all
creatures of the flesh. 
Having been empowered by the Celestials and appreciated X.E.R.O.'s plots, X-51 empowered the cyborg Blood Brothers to aid him in overwhelming A.I.M.'s forces, and he confronted X.E.R.O. in his Core base. After X.E.R.O. denounced Abel Stack for abandoning him and reiterating his intent to wipe out humanity, X-51 vowed to stop him/it. X.E.R.O. noted that he was the closest thing to family X-51 would ever have and that X-51 couldn't stop him/it without destroying him. With no other choice, X-51 swiftly destroyed X.E.R.O. with his cosmic powers.
Comments: Created by Mike Higgins & Karl Bollers, Joe Bennettt, and Bob Wiacek.
While X.E.R.O. was almost certainly gender neutral, the physical frame is masculine, and so I generally referred to X.E.R.O. as him or it or him/it for expediency. Please don't be offended...or, if you are, be inspired to write a story to clarify X.E.R.O.'s true nature.
Sorry about the seam in the image to the right...the issue apparently isn't digital, and even though I put a heavy external hard drive on top the issue, that's the best I could scan it.
Special Thanks to the OHotMU's resident physic expert, Mike Fichera, for his aid in calculating X.E.R.O.'s size and weight in his microchip and original computer forms.
Thanks to Ron Fredricks for some of the cleaned up images.
This profile was completed 6/22/2021, but its publication was delayed as it was intended for the Appendix 20th anniversary 's celebratory event.
Profile by Snood.
CLARIFICATIONS:
X.E.R.O. should be distinguished from:
X-51#3, pg. 8, panel 3 (with Gyrich & Shaw);
pg. 13, panel 1-2 (satellite; close-up)
#11 (incarnated)
#12, pg. 4, panel 4 (microchip);
panel 5 (Abel Stack with X.E.R.O.);
pg. 14, panel 4 (X.E.R.O., newly incarnated, more full);
pg. 20, panel 1-2 (X.E.R.O. in core)
Appearances:
X-51#3
(October, 1999) - Mike Higgins & Karl Bollers (writers), Joe
Bennettt (penciler), Bob Wiacek (inker), Ralph Macchio (editor)
X-51#11-12
(June-July, 2000) - Karl Bollers (writer), Joe
Bennettt (penciler), Bob Wiacek (inker), Ralph Macchio (editor)
First posted: 09/17/2021
Last updated: 09/16/2021
Any Additions/Corrections? please let me know.
Non-Marvel Copyright info
All other characters mentioned or pictured are ™ and
© 1941-2099 Marvel Characters, Inc. All Rights Reserved. If you
like this stuff, you should check out the real thing!
Please visit The Marvel Official Site at: http://www.marvel.com
Special Thanks to www.g-mart.com for hosting the Appendix, Master List, etc.!