Complex sentence: Since she has known Mamoru since kindergarten, Mamoru knows everything about her, except the object that Yūna looks at every night before bed-a photo of herself and Mamoru which she cherishes.
Split into simpler sentences: Since she has known Mamoru since kindergarten, Mamoru knows everything about her, except the thing that Yūna looks at every night before bed. The thing Yūna looks at before she goes to sleep is a photo of her and Mamoru which she cherishes.