In my novel, there is a antagonistic character named Numbers who reacts in a similar manner to those he is talking to, even freakishly so. Because of this he is easily able to become a powerful character, but I am worried if his sudden personality changes (or rather lack of a personality) will make all of his actions feel forced. How can I solve this?
Consider what happens to Numbers when he is alone. Does he maintain the last personality he spoke with? Or lose all personality whatsoever?
If the former, then he needs to avoid interacting with weaklings, or else he would become dumb, stupid, weak. When this happens, especially on accident, he needs to becoming frustrated, because it's hindering him -- he may have trouble getting back to a stronger/smarter character because he can no longer plot with ease. Allowing the readers to see this can help them understand his drive and his character switch, and allow it to seem natural.
If he loses all personality, then you have to figure out what this lack of personality is. A single-minded drive to become stronger? Is he trying to achieve some overarching goal, for which he needs to be stronger? Portraying this can help your readers see him as an antagonist, and to find his character changes believable.
Perhaps he has the ability to choose who he's mimicking, and whether he's going to switch characters (Which will give you the ability to point out WHY he's choosing to switch characters, and allow it to be believable to the reader, because he's probably going to maintain the best/smartest/strongest character during all normal scenes instead of switching). If not, and he's always imitating someone in the room, he's going to need to constantly be around a strong character that he wants to emulate -- who would be more likely to be an antagonist than Numbers, and who could chide Numbers as your typical Big Bad would a lackey for mimicking him.
Whatever you decide the reason is for Numbers to take on others' personalities, you have to explain it to the reader in a way which makes sense.
- Your character could have Giovannini Mirror Syndrome, exaggerated for the House, MD episode "Mirror Mirror." If your story is set in modern times, someone would have to diagnose the character (or take a stab at it) to let the audience know that the character is doing this for a narrative reason and it's not bad writing.
- Numbers could be a sociopath who sort of wants to get along with people, but he never really learned how, so all he does is mimic whoever is in front of him because "well, that person is doing it, so it must be 'correct.' "
- If you're writing a fantasy, he could be possessed by spirits/demons/etc.
Regardless of the tack you choose, you must show the reader why Numbers behaves the way he does. The characters around him should notice that he seems to have no core personality, and should point out that it's not normal. They should wonder to each other why he's like this. If any of your story is from his point of view, revealing his thoughts and perceptions might also help the reader understand why he's acting this way.
The other characters might try to fix him, depending on the setting and their proclivities. In a fantasy, that might mean "lifting a curse" or "performing an exorcism." In a modern setting, it would be a hospital conversation. This also opens a window into his behavior, allowing you to justify it for the reader.
Have your other characters comment on how strange and unreal Numbers is, and how they never know what to expect from him because his personality shifts so radically all the time. Your characters probably don't know what to expect from him from moment to moment; perhaps one character always sees another in Numbers' behaviour. In short, let your audience know that yes, this is deliberate.
Keep some things consistent
If Numbers is an antagonist, presumably he has some kind of goal he's trying to achieve with his interactions with the protagonists. Keep this goal in mind whenever you're writing him, and have him always progressing towards it. If he's trying to seduce one of your other characters, have him always put the moves on them, just different come-ons based on who he's mimicking. If he's stealing materials to make his doomsday weapon, have his methods for getting each item match the way whoever he is mimicking would do it.
Give him a noticeable accent or dialect, one that's stronger or weaker depending on who he's mimicking, but always there (perhaps 'e always drops 'is aitches). Give him a verbal of physical tic that always comes through.
In short, give him a core, and just change things around that core to match others.
I think that one of the important things about a character like this is that there has to be some kind of consistency. You can have the wild mood-swings depending on who he is interacting with, but I think there needs to be some kind of base-line, something which tells (or even just hints) that this character has some standard of behavior.
It's an interesting idea, but it begs the question - "What happens when he is not around other people?" If I understand the character correctly, he consistently imitates anyone he is interacting with. The problem that occurs with this is that when he's NOT around other people, he has to behave in some way, even if it's sitting in a chair, staring at the wall.
Even if the audience never sees him by himself, or doesn't see much of him at all, you as the author should have a good idea of how he behaves in many different situations. In this way, you have a greater understanding of him, and this will inform you about why he behaves the way he does.