Mosquito is Arachne's butler. His appearance is a short old man with a large pointy nose which is used to suck the blood out of a victim. He has mentioned that he can transform into his form from 800 years ago which could mean he is stronger then he lets on (but it seems he needs some high grade blood to pull the transformation off). He has a sense of honor since he let Maka, Black Star, and Kid leave during their fight since their (Maka, Black Star, and Kid) time was running out and they impressed him during the fight.

Forms: Mosquito's body changes to the way it was at an earlier age to fight.

-100 years ago: The muscles in his upper body enlarge, giving him the appearance of a huge gorilla-like torso. His legs however, remain unchanged. This form is Mosquito's toughest form and has been seen used twice.

-200 years ago: His limbs and nose extend in length, giving him a streamline, feline body structure. This form was when Mosquito was the fastest, evasive and had his nose most lengthened. This form was revealed partially when the Shibusen students had just defeated Mosquito's first form and when fighting Death the Kid and Free.

-400 years ago: In this form, with his soul wavelength swells up enormously, he takes up the shape of a young man with slick, combed hair. He is able to construct and reconstruct his body from bat-shaped darkness, and was able to sever one of Kid's arms and Free in half in barely one second. This form was when he was at his most numerous-Legion.

-800 years: This form has only been mentioned.

Attacks: Mosquito uses his needle nose for attacking, any other attacks depend on the form he is using.

-Police Stringer: Mosquito extends his nose then thrusts himself (while spinning) in order to drill his victim on his nose.

-Three Piece Police Stringer: A more powerful version of the Police Stringer but it attacks three times.

-Darkness Discord: Mosquito firstly sets up a pentacle under the target before splitting into a vortex of eight thousand bats, entrapping his victim within with destructive ultra-sonic waves.

(Source: Wikipedia)