I was originally going to do another story instead of this one, but I still need to work on that one so until that comes out, here's one I came up with:
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Your original form:

Your current form:

Name:
Experiment 000

Nickname:
(Y/N)

Likes:
Technology
Coffee
Hurting others
Winning

Dislikes:
Jumba
Humans
Your inventions getting destroyed

Info:
You were the first, you were the very first Experiment Jumba ever created and unfortunately you weren't as powerful as he had hoped and deemed you a failure.

You were also prone to bursts of rage whenever you got too angry and would go berserk.

This resulted in Jumba having to exile you to a desolate planet where you slowly began going insane.

Fortunately for you, the planet was often used as a dumping ground for parts and gadgets.

With your advanced intelligence, not only were you able to build a spaceship to escape, you also replaced most of your body parts with technology to become stronger.

You'd show Jumba exactly how powerful you were.

While you were extremely intelligent, your ship was still made out of junk and eventually broke down.

Luckily when it did, there was a planet nearby, this planet was Earth and you crashed onto Hawaii.

To your surprise, you found multiple creatures that were in many ways like you.

You concluded that Jumba had made more Experiments and you couldn't believe your luck at accidentally finding the planet Jumba was on currently.

Now you would have your revenge, it would be hard, but you always were resourceful.

Personality:
You are a mad and deranged individual who is prone to violence at the drop of a Hat.

Jumba exiling you and years of isolation filled you with spite and malice.

When you found out that all his other Experiments were spared exile, including ones weaker than you this malice and hatred only grew.

You hate being looked down on and will make anyone who does look down on you regret it.

Many who encounter you describe you as very intimidating, even the extremely powerful 627 would obey you.

You don't want to form any connections with others in fear you'll get abandoned or used again.

Secretly though, you do crave connections even if you don't know it.

Your very emotional which results in your berserk mode activating, in this state you are comparable to a wild beast.

You have no idea, but these berserk moments are actually killing you, similar to what happened with Stitch in the second movie.

It is taking longer to kill though due to your intense hatred and rage keeping you alive.

You're literally too angry to die.

Powers and Abilities:
Since you were the first Experiment, you came with a lot of their base abilities.

You have the ability to cling to walls, super strength and very durable, like Stitch, even getting run over by a giant truck won't even kill you.

You aren't as powerful as Stitch or the others though.

Hell, 625 has superior strength compared to yours and you hate that fact.

Your new cybernetic body parts do however give you a massive edge over most other Experiments though.

Your mechanical arms can morph into various weapons, like blades and plasma pistols.

You even have energy claws that can cut through anything

Your cybernetic eye has both thermal and X-ray vision.

You went all out with upgrading yourself and it really shows.

You also share a lot of Stitch's weaknesses though such as his aversion to water.

After you go Berserk and you calm yourself you'll end up very drained and tired, but during this berserk state your strength power and speed skyrocket to surpass Stitch's.

You can't think though.

Theme Song:

Your Allies:
These are Experiments and Allies you recruited to your cause through one way or another.

Experiment 629/Leroy:

Experiment 627/Carnage (He liked this name you gave him better than Evile):

Experiment 621/Chopsuey:

Dr Hamsterviel:

Love Interest:

Experiment 149/Bonnie:
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I noticed that there's no Lilo and Stitch Male Reader inserts and so here's my contribution.

Hope you all enjoy it.