Stalker is the term used Isaik as the trained seekers of objects, persons, creatures or champions. It is a specific niche in the group, these people are trained and usually tasked with bringing in new captured heroes for conversion. They are for lack of s better term the organizations version of bounty hunters. Think of them like trained and experienced dark champions who do quests for the organization.
Isaik is glancing to Rekkagg because Rekkagg/John Galloway is a Stalker, Infact he is one of the most experienced of Stalkers. As it stands Yumara has access to the top two Stalkers, Rekkagg and the most recent creation of Skalaz, if you need ingredients, materials or individuals, command one of the two to go get it.
In a way Izzabelle and Sericon are being trained as Stalkers, it is just useful skills to know if your going to kill Raveintis.