From Bulbapedia, the community-driven Pokémon encyclopedia.
Mr. Who (Japanese: ミスター・フー Mr. Who) is a character in Pokémon Trozei!. He is the inventor of the machine that drops Poké Balls onto the screen.
Mr. Who likes to collect things, particularly what he perceives as junk. He has picked up a number of Poké Balls under the nose of the Phobos Battalion, though he does not appear to understand the significance of them. On meeting Lucy Fleetfoot, Mr. Who decides to collect Prize coins instead and will trade one for a two-minute Trozei round in his den.
Rounds at Mr. Who's Den take place on a 7×5 field, with the same gameplay mechanics as other modes, with one condition. Trozeis must now start with 5, then 4, then 3, etc. The Pokémon found in each round are selected at random, allowing the player to encounter rare and even legendary Pokémon.