I mean having romantic relationship with them
I don't know for sure if it has been answered for you.
But here are the characters that can be in your personal like list, and have hearts available, but some can only be obtained with the right requirements:
- Bernard (4 hearts)
- Logan (2 hearts, missable, depending on your actions)
- Hayden (1 heart, maybe 2? not sure)
- Caleb (2 hearts)
- Arion (1 heart, maybe 2? not sure either)
- Dulrig (1 heart)
- Korg (1 heart)
- Roushk (2 hearts, missable)
- Othra (2 hearts, maybe? missable)
- Rhot (1 heart, missable)
- Ste (2 hearts, missable, depending on Logan's actions)
Tao and Scrap I think as well, but I never had any hearts with them in my previous and current playthrough so someone else can correct it for me..