Maru is believed to be the last of his kind, dating back hundreds of years before the water goddess died, causing all the water spirits to one by one leave in a panick for safety, the only one that decided to stay was Maru as he was in great disbelief she died as he was her right hand, and technically, her mother and closest friend

Context

All the spirits that left eventually lost their immortality, and current generations don't have powers at all, leaving Maru the only one that can waterbend and live forever, for this he's been living alone for about hundreds of years, waiting

He's also the only male of that time, since the goddess made all of the children female, and their was no need of reproduction, he was more of a curiosity experiment from the goddess

???

It seems like he's also looking for someone other than the goddess, but he never specifies, it's suspected to be the reason that finally pushed him to leave and look on the outside world eventually.