Marvel's Midnight Suns · Relationship progression
Friendship unlocks
High relationship levels with heroes unlock additional costumes, that hero's legendary skills, and challenge levels. Friendship is therefore a route to named skill content as well as cosmetic and challenge rewards.
How it works
The evidence says appropriate activities and gifts can boost friendship XP, while IGN describes using gifts and Havens to increase it. It does not specify the friendship threshold for any particular legendary skill.
Read the supporting references
Check the edition and source version for patch-specific details. Guide methodology.