Indie isn't a human or a monster. They're made up of binary code scattered from broken AU's that were destroyed or deleted by their creators. Fragments and shards of those destroyed AU's came together and built Indie Code.
Indie Code is a mix of all Undertale humans and monsters. They just take the form of something between Frisk and Chara, but they can shift into any form they want and look like anything because they are just binary code, a line of code.
The AU, Monachopsis Tale, takes place in a broken and shattered version of Undertale, but pieces glitch and no-clip due to it being a hybrid of several bits and pieces of AU's at once. Some parts of the AU turn into binary code before disappearing.
Is there anyone else besides Indie Code in this AU? There is a Sans but he was created after Indie was. He's not a clump of binary code like they are, instead, just small pieces of him are binary code and the rest are fragments of other Sanses.
Indie Code is a husk, unable to feel emotions, and yet at the same time they can, but the emotions they feel aren't theirs. It's the emotions of everyone that were destroyed in the AU's made to create theirs. Indie is capable of traveling beyond just the Undertale Multiverse, they can go to other realities entirely (e.g. Sonic The Hedgehog's franchise). But Monospace (Monachopsis Tale Sans) always stops them.
Will the Star Sanses or Bad Sanses ever meet Indie Code or Monospace? Yes. Monosoace hates all other Sanses because he can actually hear their thoughts and feel what they feel (due to him basically being several Sanses in one) and Indie is incapable of forming their own opinion on the matter, but all the people in their code scream at them that both sides (Star and Bad Sanses) are not to be trusted.