Unmarked spoilers are ahead, per Spoilers Off rule.
The very end when Riki comes back to be with Iason.
During Riki's second coming out party in Eos, after he had been affected by the D-Type ring as punishment for bad behavior, he's too tired to hold a glass of water and drops it. Iason then picks Riki up onto his lap, drinks from another glass, and kisses the water into Riki's mouth to drink. Riki is touched. (The other elites watching are not.)
YMMV with that one, considering that situation resulted after being brutally punished, which is not something I would consider heartwarming in the least, especially when you consider that almost everything Iason does, both the violent and affectionate, is specifically designed to make Riki completely dependent on him. He's basically engineering a case of Stockholm Syndrome.
By that logic, it can be argued that any moment considered heartwarming between Riki and Iason has a dark underlining that ultimately ruins it.
While I think that Iason's manipulations of Riki is true for the most part (I added those sections and tropes myself), Iason does have a few moments when he acted out of sincere concern for Riki. The first real one is when he moves Riki to Apatia after a furniture attempted to murder Riki, which prompted the other Elites to use that incident (and what Riki learned from it) as an excuse to call for his death. With that action, it shows Iason realized Riki could no longer reside in Eos safely. Another defining moment of Iason's is when he spared Guy: that happened out of compassion and not just an attempt to impress and manipulate Riki. The moment mentioned above-when he gave Riki water-was also genuine compassion, but probably didn't matter due to the events before and following immediately after when they got home and Iason showed himself as the sadistic bastard he actually is to Riki.
