Merle
is a 13 year-old catgirl with an obsession: Van. She is always clinging
to him, hugging him, or licking his face. She's a very possesive person.
When she's not attaching herself to Van's arm, 
she's
going through Hitomi's stuff, taking it and/or tossing it around for fun.
Merle likes to get attention from people (especially Van) When she doesn't
get it, she gets jealous of other people who do. She's very jealous of
Hitomi because she knows Van has a thing for her.
Merle doesn't seem to "help out" much in battles (or in anything, for that
matter), and although 
sometimes
she is compassionate, it soon fades back to her usual, annoying self.
I'm sorry if you found that last bit offensive, it wasn't meant to be.
I actually like this character, but even her greatest fans must admit:
she's annoying.
Like a cat, she's very curious, and constantly snoops about, always asking
questions, getting on people's nerves. At first she and Hitomi fight a
lot, but in the end they become pretty good friends.
Merle has been Van's best friend since he was really little. She affectionately
refers to him as "Lord Van" and obeys him even when he doesn't mean to
command.
Merle has no real family, and was taken in by the Fanels when she was a
baby. 
Although Merle has rather annyoing traits, she provides most of the humour
in the show, and in the end turns out to be a great person/cat to have
by your side. And unlike most cats, she is loyal.