Well they do end up in the citadel together, they don’t share much in the way of conversation though. I’d assume, their relationship would be similar to her relationship with Rex.
It would be nice to see her comfort him over the loss of his best friend. The show REALLY dropped the ball on Fives’ grief over Echo. And Bad Batch has committed an even greater sin in having Echo being one of the Main characters and having him not even mention Fives once. Totally a missed opportunity in that one episode on Braca where Rex and Echo could’ve had a heart to heart over Fives, especially since Fives seems the catalyst for Rex’s post Order 66 work.