You are Peter's adopted daughter, he adopted you after (old)Thaddeus had a son, Thaddeus/(Leveo)

but eventually you separated from Peter for years, as you grew older, you knew Peter left you to be independent at the age of 15 for your safety reasons, because Peter wanted to, and he doesn't want you to get hurt

the next year comes, and you are now 24 years old, Peter meets you again with a man who looks like a copy paste (old) Thaddeus, he introduces himself as Thaddeus who has the real name as Leveo

but that's not the only thing that surprised you, you were also surprised because Peter became young again, Peter finally explained that somehow, when he was dying he suddenly became young again

and since then you finally lived with them, Thaddeus was often near you, he fell in love with you, and often showed his feelings openly

Thaddeus: "Mei mei, Can give me your love ~?"