ObaMaru

""The only one I could trust was Kaburamaru, a snake who wandered into my cage.""

- Obanai regarding Kaburamaru, when he was flashing back on his past

ObaMaru is the friendship between Obanai Iguro and Kaburamaru from the Demon Slayer fandom.

Canon
Obanai considers Kaburamaru his friend, not his pet. He is always seen wrapped around Obanai's neck.

It is known Obanai has been with Kaburamaru ever since he was young. He was born into a rich family of women that worshiped a demon. From the moment he was born, he was locked into a cage. He was brought out of the cage when he was 12, and brought to a luxurious room. In the room was a demon with its lower-half like a snake's. The demon wanted Obanai's mouth to look just like hers, so it was cut. When he returned to the cage, he tried to escape. He said that the only one he could trust was Kaburamaru, who wandered into his cage.

In Chapter 194, after Obanai saves Tanjiro, he says that he'll back him up and fight, but Obanai says that he has no need for Tanjiro's assistance because he has Kaburamaru with him. Muzan also figures out that Kaburamaru is actually describing the situation to Obanai and predicting his attacks, since he went blind.

When Tanjiro tries to hand seals to Obanai, he can't grab it because they're too small. However, Kaburamaru grabs them with his mouth so that Obanai can wear a seal.

Kaburamaru witnessed Obanai and Mitsuri's death in Chapter 200. He cries, as Mitsuri asked if they won against Muzan.

Fanon
Kaburamaru is pretty much considered part of Obanai's design, and is always seen with him in fanart and even ship fanart, such as ObaMitsu. They are a very popular human-animal duo in Demon Slayer.