Spoiler
They’re the same person , he’s the illegitimate son of the former duke that was exiled or rather not even acknowledged
He’s with blondie for revenge
The blondies father raped ain and abused him when he was working as a servant in their estate
Even while blondies mother was pregnant with him ,the father continued with the rape…ain then promised to have the top to himself… as far as I know he’s planning to kill him but later changed his mind (but he’s still toxic af as he’s drugging top and stalks him etc ,Oh also SA him in his sleep )
Then ain went to the his dad’s estate and to survive his stepmom rapes him constantly till he kills her.
His life is rly sad
And you’re right he is using top for revenge

Someone spoil me please! Who is this Ain and is he Ian?
I have two theories. One - Ain/Ian was a young ward in the south under the Blondie's father. Who showed the northerner the ways of love. That's why he likes to bottom and why he says he could never not like Blondie. Second theory- Ain is an illegitimate child and a half brother to Ian. His life was cruel and he's on a revenge path and is using Blondie.