Personal goals.
This one is sort of a hiding-in-plain-sight trait that's easy to miss, but part of characters being interesting is that they have personal goals. They don't have to be big ones, they can be little ones, such as getting better at a specific skill or hobby, but they still have something they are working for.
This is part of how slice of life and progression fantasy can overlap. You just need characters whose hobbies include training because they like the feeling of pushing their bodies and their limits, and like having a good, friendly spar.
But this should not be their only goal. I love Goku, but a good Slice of Life protagonist he is not.
Relationships, dealing with family, dealing with the *other* person's family, all of these interact, but a character who does not have something that they, personally, want to do will tend to come out a little flat.
Perhaps a guy just wants the nice quiet life of a farmer and sets about making that happen. He travels out to the edge of civilization, meets a cute girl, starts getting to know the locals of the nearby town, and everything is going great.
Then shenanigans happen that make life difficult. I've now introduced you to "Beware of Chicken" (one of my favorite mixes of SoL and PF).