You can unlock one of the following characters.

Character	How to unlock
Archetype Earth Arcueid	Beat Arcade Mode with all characters.
Mecha Hisui w/ Kohaku	Beat Arcade Mode with Neco Arc.
Neco Arc Chaos	Beat Arcade Mode with Hisui and White Len.
Neco Arc w/ Mecha Hisui	Beat Arcade Mode with Miyako.
Tohno Akiha Uniform Version	Beat Arcade Mode with Tohno Akiha and Akiha Vermillion.