The theme behind Udyr's Ultimate Spirit Guard Udyr[S|L] skin is one of the possible outcomes for Runeterra. In this future, Udyr reaches inner harmony and conquers his rage during his time with the Ionian monks. He then guards the Hirana Monastery from demons and corrupt monsters that would use the Temples power for the destruction of Runeterra.
This future may also depict a possible outcome for Ao Shin.
His appearance and clothing style parody Japanese monks' appearance, with comically oversized & incorrectly worn prayer beads, hakama, geta, samurai's kote, & stylized non-Japanese lungi, yet without the kesa robe.
He also bears a striking resemblance to the Sura Master NPC from Ragnarok Online, if one doesn't count for the hairstyle.
Spirit Guard Udyr's Level 5 Bear Stance has a similarity to the Street Fighter character Oni, one of Akuma's alternate forms. Both have bluish glowing skin and aura, a large prayer beads, bear-claw shaped hands, exposed upper bodies and gruffy hairstyle resembling flames.
Using a Dance, Laugh, or Taunt emote command with Spirit Guard Udyr summons whichever spirit corresponds to his current stance. His Dance will summon all four spirits in sequence once any given ability has reached rank five. His Joke summons Urf, which finally explains where Urf's spirit went after being killed by Warwick.
This skin represents his state as a monk after finding his inner harmony and conquering his rage completely, even when fusing with one of the four spirits. This stands in strong contrast to his non-ultimate skins which represent his feral nature and somewhat still-uncontrolled rage.