Winchester Model 1887 Shotgun – ★★★★★ Shotgun
Aka M1887. She's a shotgun, so she will do shotgun things shotgun-y (read: facetank with her armour), but is not particularly impressive at it. Her stats are okay (but worse than Elphelt's), her tile buffs are the weakest of all 5* shotties (and most 4*!)… and her skill is downright useless, because it has a ridiculously long 15 second cooldown, yet its strength depends on the amount of links still alive at that point (which won't be many).
As far as free SGs are concerned, a minor upgrade over 97S if you need armour over buff tiles.
She can be used for farming UMP exos during Deep Dive, but raising her *just* for that is probably not necessary – the exos drop from two maps, she's only good for one of them, the UMP exos aren't a priority to farm, and they'll be added later as permanent drops anyway.