I don’t think Yusa gained her powers due to Misa dying and wanting to see her again. I think her desire was there from the start: Yusa, being an airheaded imouto who was the opposite of Misa, wanted to become a person like Misa, perhaps subconsciously. In a similar way that Yu was always thinking of others, Yusa was specifically always thinking of Misa. This manifested in that she can now actually become Misa. Maybe if Misa never died, the “conditions” would not have been right and Yusa wouldn’t have developed powers.
Perhaps her lack of character is her character in that she subconsciously uses Misa as a crutch so that she never has to think or worry about dealing with anything. Or maybe she’s just a bad character.