Kingcup was a buck from Cowslip's Warren. He is a minor character who was mentioned when Strawberry was trying to find him, but he wasn't in the burrow. It is theorized that he was killed by the Shining Wire.
Miniseries[]
Strawberry calls for Kingcup to meet Hazel and Fiver, but he is never seen, and she is visibly saddened to see that he's gone. It's implied that he's already been killed by a snare, and it's possible that he was Strawberry's mate before his death, making him the miniseries' version of Nildro-hain.