Nickel Buddy is a nickel owned by Mr. Krabs. It only appears in the episode "Bubble Buddy."
It is a nickel of standard design. Specifically, a small, round, silver coin.
Role in episode
When the angry mob of Bikini Bottomites at Goo Lagoon attempts to pop Bubble Buddy, SpongeBob asks if any of them can relate to having a special friend. Mr. Krabs looks down at Nickel Buddy and promises never to spend it, leading to others showing love for their special friends.
However, this love is short-lived, as they continue their chants to pop Bubble Buddy.