Sage isnt a Sentinel (You can counter argument on that)
she doesnt belong more to the of a Sentinel by Riot themself in the game, (It's impossible to give geniune counter argument to that part) she fit a little bit more to description of controller than the description or even an initiator
Sentinel
- emphasis on supporting the rest of the team through providing intel, (She dont, only wall but not gonna be use like that)
- anchoring down sites (prob the worst anchor in the game since anchoring mean usually be alone and half of her kit require to not be alone)
- preventing the enemy from flanking. (You have nothing in your kit to do that, and use the wall to do that is pure int)
Controller
- assist their teammates by cutting enemy vision, (In atk the wall is used to do that but also to block bullet)
- taking control of an area, (Slow orb, Wall)
- and forcing enemies into choke points. (Wall is the best spell ever to force that)
Initiator
- initiating attacks on site, flushing enemies out of hiding spots and corners in order to assist the team when charging into battle. (All her spell are good for that, Wall take space Orb lock trap people, heal and rez are more easier to make it work in atk because you tend to group more with team than in defense)