Reindeer is a minor Friend that appears in Chapter 7 of the manga.
Reindeer seems to be a Friend brimming with optimism. In the middle of a snowstorm, she can still exclaim words of encouragement and tell stories as without any adverse affect on her body.
She seems to be a bit of a ditz as she suggests slightly superstitious ways of battling off the cold weather instead of actual physical help. Despite being in the snowy mountains for a while, she does not know how to go back down on foot.
Role in the Plot
Volume 1 Chapter 7
Reindeer finds a freezing Serval and Caracal in a cave in the snowy mountains, checking out why there were Friends there during the snowstorm, as she's never seen Friends in need of help in the snowy mountains before. She offers them ice cream as it's the only food she has with her, which just serves to make the feline Friends even colder than they were, but had no choice as they were already hungry. Caracal asked her if she knew how to go back down the mountain, but explains that she doesn't as she never went back on foot.
She then tries to demonstrate a slightly "power of the mind" way of battling the cold, by shouting lines to the effect of "I feel warm" while taking in the snowstorm. It doesn't work for the two felines.
Out of options, she tries to go into storytelling to the two Friends to keep them occupied, which for some reason sounds like she's telling the story of finding them. Before she gets even past the introduction however, Caracal and Serval have already frozen and are in danger of physically withering.
Fortunately, the snowstorm stopped soon after, which allowed them to finally slide back down. Serval wakes up during the ride.
After descending, Reindeer's whereabouts are unknown.
Serval and Reindeer seem to be acquainted prior to the events of Chapter 7.