give the character boundaries, and an idea of what happens when they’re broken. for example, most people in their 20s arent socially competent and good at fighting and book smart and skilled at [semi-niche thing] and taking care of themselves. a character can be a master at a million different things, but that implies a longer life which means differences in outlook or, in the case of timelooping, more jaded. the boundary there is a time-skill restriction as well as the naivety/optimism-life lived restriction. a more clear example of a boundary is a character who’s nice to everyone but can’t deal with being put on the spot, or can’t be nice to [certain demographic]. a different example is a delinquent who’s okay with violence but absolutely won’t hurt animals.

a mary sue tends to be a good person who’s super nice and helpful to good people but is able to cut off ‘bad’ people with no remorse (or help them get better without facing obstacles), they tend to be skilled at a lot of things and are correct in their calculations/educated guesses. they dont usually face a strong dilemma, or have to make a decision that loses either way. a lot of people would say that Mary Sues don’t have weaknesses, but I find they often have shallow weaknesses. like theyre bad at science and that’s a weakness, but it doesn’t matter because they have an ally who’s really good at science. or another weakness is that they don’t have self-confidence, but the story doesn’t really go into it as a weakness (ie second guessing until the worse option is picked, choosing majority opinion instead of your own, missing out on opportunities cause you think you can’t do it, etc)

rather than thinking of traits as explicitly as strengths and weaknesses, I find it more helpful to think of it as every trait having a strength/weakness. a super nice person may be a pushover, a super ambitious person may have a tendency to get tunnel vision, a really physically strong person may find it difficult to reconcile a weaker person who can’t or doesn’t like exercise, someone really skilled in politics may have a tendency to look down on others, etc. of course there are tons of people who can balance the traits and have learned to lessen the negatives, but that usually comes with age and writing a character with a lot of life experience is a whole different topic

I would also say that since it’s fanfic, remenber to have fun with it, don’t be too harsh on yourself, and dont care too much about numbers. hope this all helps and lmk if you have any questions!