someone dressed up as Alanddin is going to visit us为什么是dress

过去分词作定语表被动,dressed up as Alanddin 相当于定语从句：
who is dressed up as Alanddin 意思是被打扮成为Alanddin.
dress sb up as sb else .(使)装扮成另一种样子
He dressed himself up as Father Christmas.
他打扮成圣诞老人.
She was dressed up as a lady of high society.
她把自己打扮成一位贵妇人.