Rain D. is a large, fast-moving enemy with moderately high health. At 100% strength magnification, his headbutt attacks deal about 5,000 damage each. Rain D. also notoriously has an extremely high knockback count and deceptively long range along with a fast attack rate, making him quite the threat when guarded by and supporting enemies with few to no knockbacks.
To stall Rain D, use a few units that deal enough damage to knock him back while outranging him, such as Paris Cat, Sexy Legs Cat, or Dragon Cat, so that he doesn't quickly shred your defenses before you store up enough cash to deploy heavier hitters such as True Valkyrie Cat or Ururun Cat. Sending thecheapestmeatshields (75¢) on different intervals can help to effectively stall Rain D. while accumulating cash.
His knockback count can be annoying or helpful in the right situation. If constant damge is dealt to him, he can be kept in a knockback cycle until he's defeated. A good example of units that can do this are Surge Attackers, such as Ritual Saint Kasli and Kasli the Bane.
His Japanese name, Nakai-kun, is derived from the word tonakai, meaning "reindeer". It's also a reference to Masahiro Nakai, who had a television program named Nakai-kun Onsen and, like Rain D. in the Japanese Version, likes baseball and singing.
According to his Japanese description, Rain D. may actually be a person wearing a reindeer costume.