So as I am working on my project, there is a part that is really important: It is a character that is in search of their friend. Both characters are very important to the main sequence of events. The character that has been looking for them has made it their mission after finding their friend has gone missing. How would such character develop? Would they become less trustful of that person after they left and less trust or reflect on their past and in turn become more close to them in a sort of distant bond? Just wanted to know.
Separation of two once-close characters usually means both of them are going to experience two separate developments as well. I think it would be pretty interesting if the two finally met each other one day and found that their counterparts had become something they don't recognize anymore. There's a lot of potentials to make the story rich, exciting, and fresh.
Possibly. The hook would be that the missing person isn't "missing" at all but has deliberately disappeared and doesn't want to be found. This happens quite a bit in real life when people just pick up and leave without telling their loved ones. The loved ones think the person has been killed or kidnapped and run through all the usual tactics and stages of grief. Turns out, the person was just a selfish douchebag and put their loved ones through hell for no reason. And then the next hook--which probably happens in fiction more than in real life--is that the person HAD to disappear and not tell anybody to protect their loved ones. I feel like there's a zillion stories where this has happened, but I can't think of any at the moment.
Disclaimer: I find the new gender-neutral pronouns to be confusing. I still don't know if your outline-of-an-outline above is talking about two people, three people, or more. With that out of the way ... Perhaps there's more in the story so far that hasn't been conveyed in your summary, but if Person A has "gone missing," why would Person B become distrustful of Person A, OR become closer to Person A -- if it is even possible to become "closer" to a person when you don't know where they are, or even if they are alive?
Even in real life some people feel that they HAVE to disappear, even if the mundane reasons aren't fodder for a story or logical to the rest of us.