Both are major antagonist and the last opponent in their series, that both focus on a group of pepole who are some kind of "Special human" (Adnoramlites/Half-bloods) and are really, REALLY old (Iihiko is around 5000 years old/Kronos is older then all the gods) 

Their role in the story involve another important person (Shranui/Luke) and them more or less taking over their bodies, with said person getting their traits in the process (Shiranui got Iihkos horns and eyes/luke got cronos golden scars). Said person is also from a settelment in the wilderness (Shiranui village/Hlaf-blood camp) 

Both, long ago in the past, had a battle with a god like being/a group of god like beings (Najimi/the gods) who later where responsible for making the faction the main cast are apart of (Najimi was the founder of Hokinawa academy/the Gods are the reason half-blood camp exist) 

They where evantually defeated and also left the body of the person the possesed, by the main group. 

They have ties to a certian clan of pepole (The shiranui clan/the gods and half-bloods, who are in a way all related) 

Contrasts 

Ïihko is rather arrgoantand even joking, like always calling things fresh while kronos is extremely serious and full of hatred 

iihko beat Najimi while Kronos lost to the gods 

Shranui didint want to be Iihkos vessel but was forced to, while Luke chose to be Kronos vessel, only truning on him in the last minute. 

Shranui lived after iihkos defeat while Luke had to die to Get rid of kronos. 

Iihkos, even after being beaten, had a weakend echo of himself left, even showing up in the last few chapters, while Kornos was scattred over space, never to reform 

Iihkos main ability is that every damage he does, cant be undone, while Kornos has achellies curse, wich Ignores damage 

A few minor things 

Iihkos, in his previous life was a warior with simmilar armor and sword to the spartans, and Cronos is obvs based of greek mythos 

Both in some way serve as an allergory for a certian concept (Iihkos is a man from another time, showing the importance of progress/Kronos is, in some ways, a showcase of generational abuse with him being the father of the gods and thus ancestor of the half-bloods and wanting to kill them all)  

Iihko was immortal wich made him the way he is, while Percy gave up his immortaity after Kronos was beaten. 



Iihiko vs Kronos (Medaka sv percy jackson) "From another time" credit to ohlookanidot for the TN and connections below