Misabel is a depressed and paranoid young woman, prone to crying jags and paranoid fantasies.
Misabel is introduced in the 1954 novel Moominsummer Madness, taking refuge from the flood with Whomper on a wood-shed roof.  The pair accept Moominmamma's hospitality, and they accompany the Moomin family when they find the deserted theater.
Misabel also appears in a 1956 Moomin comic strip story, "Moomin Mamma's Maid". Mrs. Fillyjonk makes Moominmamma feel irresponsible for keeping a disorderly house, so she advertises for a maid. Misabel answers the ad, bringing along her dejected dog Pimple. Misabel works tirelessly, but she makes everyone in the family feel bad. The Snork Maiden curls Misabel's hair and gives her a makeover, which cheers her up tremendously. Misabel doesn't appear in the series after that.
When she first appears in Moominsummer Madness, she's described as "a fat little Misabel", so it is possible that "Misabel" is the name of a species as well as the individual. (This would be similar to Moomintroll being both the collective name of the species as well as the main character; ditto "the Snork" and "the Hemulen".) If this is the case, then the would-be actress in Moominsummer and the maid in "Moomin Mamma's Maid" may be different Misabels.