There are in total 5 different characters you can play with but you start first with Joe Blo and then you have to unlock the other 4 by either buying with tCrystals or Vouchers after you played the required escort mission mike is one of the second characters you can get in the escort mission and can be compared to scout from team fortress 2
On day 2, Mike will be found in the corner of the sewer map, you have to escort him to the end of the sewer map to get the ability to unlock him with Ttickets, his ability is spawning a turret like baseball machine that shoots baseballs at the closest zombie.
On day 9, you have to escort Hubert across the sewer, again after rescuing him you can unlock him using Ttickets, his ability is a trap that pulls zombies to a dumbie that is above a bear trap.