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Keep the Reward is this for more explicit heroism. Outside that this might be tropeable if you can find a meaningful pattern for it; at the moment I'd look through the Motivation Index for what makes them want to work for free and file it there. (IE, Doing It for the Art, For Happiness, Good Feels Good, It Amused Me)
The part that makes me think this is tropeable separate from that is that it can be deconstructed. One Punch Man is a good example where working for free had him living in a cheap apartment in a literal disaster area, and any working artist will tell you that this is a good way to lose months of work to well-intentioned projects that fail to make any money and selfish individuals looking for free art.
Edited by Scorpion451
A character likes their job so much they do it for free.