Mo keeps a photograph of his wife—a.k.a. Meggie's mother a.k.a. Teresa a.k.a. Resa—under his pillow at night. After Capricorn's men take Mo away from Elinor's place, Meggie creeps into Mo's bed to sleep that night and "put her hand under the pillow. Yes, there it was […] a photograph. Meggie drew it out. It was a picture of her mother; Mo always kept it under his pillow" (9.2). For Mo, this picture symbolizes his attachment to his wife and his hope that he'll find her again.
Dustfinger has a photo of Meggie's mom too, but it means something different to him. We're immediately clued into this difference because he keeps his photo in his backpack (31.35), which is not only a different place from where Mo keeps his, but also less intimate (Mo and Resa have shared a bed; Dustinger and Resa have not). In Dustfinger's head, it makes sense to try to steal Mo's wife. He says:
He's taken a whole world from me, why shouldn't I take his wife from him? (43.27)
Dustfinger's attempts to get close to Resa represent the family he's never had, that sense of stability and belonging that always seems to elude him. Mo is as easy a target to blame for Dustfinger's endless outsider status as anyone else.
We don't want to put Teresa/Resa on a pedestal or say she's the perfect woman or anything, but the fact that both Mo and Dustfinger long for her—as evidenced by the fact that they both carry around her photo with them—shows us that she has made pretty big impressions on each of them. Both are based on longing, though for Mo, it's a desire to reunite his family and rekindle his relationship with Teresa, while for Dustfinger it's all about gaining something he never had.