No real quote I can give, as only his backstory is given to us in canon, and even then, that's just in Youmu's bio. He is mentioned in Yuyuko's as well, however.
He was the only one who knew about the true story of the Saigyou Ayakashi, but he figured Youmu would learn of it in time. Well, she did, but it was some poor timing for it.
He was Yuyuko's half-ghost gardener for around 300 years before he took off to become a hermit, leaving young Youmu in charge. "Though Youmu herself has no clue where her master is, she thinks of her master's absence as a lesson from him," says her bio. I can't quite tell if that's inspirational or just depressing. Seems more like the latter.
ZUN seems to be okay with the fan interpretation of the character, and it's rather consistent. In terms of appearance, I mean.
Might not have been as devoted to his job as Youmu, but few Gensokyians are as devoted as Youmu anyways.
Depictions of how he and Youmu got along tend to vary. http://nazr.in/1p4
Nobody knows where he is but him. Due to that, I'm tempted to slip him in with, say, some PC-98 characters in the future. Y'know, he could very well be a full ghost by now.
Judging by Youmu's unwavering dedication to Yuyuko and her superb sword skills, he did a good job of training her. Just vanishing like that can't be that good for her, though. If he's her father/grandfather (nobody seems to be sure which), the one guiding her through life, then of course she's so dedicated to Yuyuko. While she may be Yuyuko's servant, Yuyuko not only fills the role of master, but also the role of mother and father figure, in a way. Of course she's loyal to the end, if that's the case. They did note that Youki left Youmu in that position while she was rather young.