The Deer Spirit is a character that appears in Teenage Mutant Ninja Turtles 2012, during the series' "Country Saga," at the start of season 3. He is a do-gooder who never speaks, however, he still helps/guides the Turtles on their mission(s) in the country.
History
The Deer Spirit first appears in the first episode of Teenage Mutant Ninja Turtles season 3 titled, "Within The Woods". During his debut, he takes the form as a normal deer in order to watch over the Turtles, without them figuring out his true form or purpose. Throughout the episodes in the "Country Saga", he continues to spectate in his normal deer form, appearing and disappearing ever so swiftly.
In the 60th episode of Teenage Mutant Ninja Turtles titled, "Vision Quest", the deer (in deer form) attacks Leonardo, in order to make him realize that the Turtles have to find their spiritual selves in order to defeat the Shredder, whose back in the city. Leonardo sees this and tells the Turtles that they have to hone their skills in the woods. Once their training begins, the Deer reveals his true form, and appears in front of our Turtle heroes. Raphael suspects that The Deer is a dangerous mutant, so he rallies the team to attack him. Despite Leonardo's commands to stop attacking, the rest of his brothers continue their assault, however they have no luck. Without saying a word, The Deer swiftly dodged their every hit, making them land blows on each other. Eventually, the Deer disappeared, without the Turtles ever knowing where or how he vanished.
Shortly after, the Turtles received a spiritual message from Master Splinter (presumably from the powers of the Deer), which lead them to beginning their individual spirit quests.
While Raph, Mikey, and Donnie faced their challenges alone, The Deer decided to accompany Leonardo, who's also battling a severe knee injury. As Leonardo's quest begun on a bridge, spirit versions of Foot Bots appear and attack him. The skilled ninja easily fends them, however, after he does, The Deer, in his true from, appears before him. Leonardo, startled, looks up at the Deer in shock, and in curiosity. Just then, the bridge suddenly starts to shake, leading to Leonardo falling off! Being the ninja he is, Leonardo was able to recovery, and escape unharmed. As the leader of the Ninja Turtles made his way through his quest, The deer (this time in deer form) appeared in front of him. Once Leo realizes him, he walks up slowly and asks - "What is it? What is your lesson?"
Before the Deer could respond, the Spirit version of Shredder approached and said- "Lesson? You've already failed!" Leo engaged in an epic battle with Shredder that wasn't going his way. Because of his knee injury, Leonardo thought that he'd never recover, therefore he was doomed for failure. At this time of despair, The Deer (still in deer form), floated in the air above Leo, and without speaking, the Deer was able to show our fierce leader that that pain he thinks is holding him back, is only in his mind! After Leo realizes this revelation, Shredder suddenly grows bigger than the mountain they stood upon! However, Shredder wasn't the only thing that grew. Leonardo's confidence was restored, and he was ready to face his challenges. To cap off their ultimate adventure, The Deer reverted back to his true form, and merged his spirit with Leonardo's! Because of this, Leo's knee injury was gone, and he was able to take down the Spirit Shredder, and complete his quest. Even though The Deer has not been seen since that day, we already know where to find him. And that's Inside the spirit of Leonardo.