Marvel's Daredevil: 31 Easter Eggs You Probably Missed
15. Stone
At the end of Episode 7 Stick talks to someone, seemingly reporting to him. Some suggestions said that this could have been a superior figure - someone like Master Izo - but it seems far more likely that it was in fact Stick's favourite student Stone.
That's corroborated by the fact that Jasson Finney is credited as playing Stone, who is another character created by Frank Miller. The fact that he's been actually cast suggests he's likely to turn up again in the second season, and with a move towards more ninja lore, he'd certainly fit.