He shows up in Japan looking for Shampoo, even though he always fails to defeat her in a
fight, but Cologne tells him Shampoo is going to marry Ranma.
So he challenges this new rival for Shampoo's affections to a man-to-man fight
(a troublesome point for Ranma). When Ranma defeats him, he returns to China and seeks training
at Jusenkyo, where he literally walks into Yaazuniichuan, a spring where a wild duckling drowned 1300
years ago (if there's anywhere in the world a duck could drown, it's there). He returns to
Japan with a supply of Yaazuniichuan water to turn Ranma into a duck, too, but (of course)
fails.
Moose hates Ranma, and would dearly like to do away with Shampoo's "husband".
He's skilled with a lot of weapons, which, by the way, appear from his sleeves (literally), or from his wings, when he's a duck. It's a cool trick, though he DOES run out of ammo sometimes...
Mousse is basically a nice guy, just desperate to get Shampoo. He's so desperate he does crazy things. If only he didn't mistake Shampoo for other objects and people, maybe she'd take him into consideration.
As I said before, he has been trying to get her to agree to marry him, but it
looks like that will never happen. First, according to the village edict, he has to defeat her
to get her to marry him. But, as we know Shampoo....And secondly, Shampoo is in love with
Ranma. 
But sometimes, Shampoo seems to take pity for Mousse, especially when he risks his life for her.
Sometimes, he asks Akane for help to get Shampoo from Ranma (as if Ranma owns Shampoo!...Heck, you know what I mean...). I don't know if he teamed up with Ukyo...
The only advice I can give to this guy is...Watch where you're going, man!
Notes about the name
It's yet another *sigh* bathroom product^^ But his name really means "wash + thread"