For any character you need to have an answer to the questions:
- Why do they adventure?
- Why do they stay with the party?
- Why does the party accept them?
Cowardice means all 3 of those questions need a stronger answer.
And loyalty is a good answer for all 3.
Just be aware the roleplay could be repetitive.
"I'm scared!!!"
"Suck it up and keep going"
Might get old fast for some people.
OTOH "Repetition makes it funny" is a thing in comedy, so, choose your audience and accept that maybe a certain amount of the character's dialogue doesn't need to be said at the table every time.