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 and drinks from another glass. Iason then kisses the water to Riki for a 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.