Ari & Penny's relationship in stages for people who aren't me and Melly-Mun and don't know this yet lmao (Sophie & Penny belong to @wellfireaway )
Ari's demonic friends find out he's only alive because his Father manipulated the Charmed Ones into rescuing him from The Ice Cream Truck, his mother uses that to get to her ex/his Father but manipulating him into attacking a Halliwell
Ari being a moron attacks Sophie Mitchell only to be stopped, his Dad basically looses it and gets his ass beat trying to save him and convinces them to bind his power instead of vanquishing.
Ari's powers are bound and because of 19 year old teen boy angst no longer affecting his advanced intelligence he starts to Chill™️. His powers were bound however in a way that his witchy ones would slowly unlock as he proved himself not a threat.
At this time Penny is married to her first husband and become close friends, Penny seeing the potential for good in him and doing what she can to help nurture that.
Ari leaves to travel and try to "find himself" and try to figure out what he wanted from life now that he wasn't living a demonic life.
Eventually he hears Penny is getting married again and come back for the wedding, having realized his feelings by this point but not saying anything because he's just glad she's found someone again. He's also taken an empathy blocking potion at this point to try and hide his feelings even from her and her cupid family members.
Penny eventually leaves her second husband, and Ari waits a month or two before finally telling her about his feelings.
They start dating and eventually are married with a daughter.
When their daughter is almost 6 years old Ari has a bit of a breakdown, all of the horrible things he's done front and center in his mind after one of his old demonic friends attacks his daughter as a way to try and sway him back to the evil path(via manipulation from his Mother)
After/because of this he just up an vanishes a few days after the attack, all that's left being a note saying he's sorry but their better off without him there. He's done to much bad and they were safer with him gone
Kieran is livid, but Nicholas understands and defends his son.