If you cowrite with someone who is in a relationship with you, you will be best protected if you are with someone who is intellectually and emotionally honest with themselves and with others.
Authors who are not so honest with themselves often become vampire authors, siphoning other's creativity into their work, like a blood transfusion.
The vampire authors will often drive a spike into your creativity, bigtime and most often will not mean to. They don't see what they do because to admit their tendancy is to admit that they might be doing someone harm and of course they would never hurt anyone, they are good people. In fact, how dare anyone imply that they are harmful in their actions? At that point they will attack anyone who attempts to point out this habitual behaviour of theirs.
They may not even notice when they rewrite history to try to make themselves more creative, more interesting, more 'the real author', the genius, whilst relegating your contributions to the 'they helped' pile, like an actor thanking the 'little people' who helped them gain this attention or that award or some adulation that is desperately craved. It is very much addictive behaviour.
Most coauthors like that don't realize that this is what they are doing. Memory is maleable. They remember things very differently than you do. After all, they are the talented one who let you put your two cents into the work. They 'let' you help them and it's only help, not serious work. This intellectual stance lets them gloss over whatever you do for them, and smother it in a thick sauce of their style; making it theirs.
This is the most insideous form of plagerism there is, because you cannot say 'That was my idea/plot device/character/culture.' without sounding petty and vindictive. Vampire authors count on people being too nice to say anything.
Of course this behaviour eventually becomes obvious, even to the most die-hard fan, as the vampire author either drains or destroys their co-authors and if they become desperate to get the next creative fix become more obvious in their attempts to dupe the next newbie who draws their attention.
New authors need to be aware that there are other authors out there doing this kind of thing. Not just authors, artists of all sorts. But I have seen this behaviour in the writing world. It is something to be aware of, and to be careful of. Even peers are sometimes subject to this kind of intellectual vampirism, and it doesn't sparkle.