A tame two-tailed blue squirrel - possibly native to Solo - Rapappa was nearly always inseparable from Deck, and usually perched on his shoulder or on the top of his head. The only time he didn't follow his owner was when Deck impetuously plunged into the sea on the planet Flag Star - showing that he was possibly the brains of the operation.
When Deck moved up to co-pilot in the Ideo-Delta, Rapappa went with him, even getting a spacesuit (which was a bit odd, as they had trouble finding one for Deck and couldn't fit one that fitted Lou... still, it's cute, so we'll let it go). Rapappa was killed along with Deck when the Ideon was destroyed - distressingly I can't spot his astral form later, which is a bit harsh.
Oh, come on... he's on the Ideon for about half the series, he gets a spacesuit, and he probably does more than Bento. It's only fair.