Malbonte: how to romance them in Heaven's Secret

Short answer: cheap, and all in one season

If you want a full romance in Heaven's Secret without draining your stash, Malbonte is it. His route has 10 steps, 7 of them are free, and the three paid ones add up to 77 diamonds. Nothing happens before Season 3: the first choice is "Stay silent." in Episode 2, and it costs nothing.

The three choices that cost diamonds

• Season 3, Episode 3: "Touch him and calm him down." — 20 diamonds.
• Season 3, Episode 4: "Stay here." — 31, the priciest step of the route. The same episode also has a free one, "No, absolutely not."
• Season 3, Episode 7: "Kiss him back." — 26.

The rest is free all the way to "Ready." in Episode 10, the finale of the season. Episode 8 alone gives you two free steps in a row.

Pay or wait?

With 77 diamonds total, I'd just pay as you go: none of the three is a big hit. If your stash is empty, play Season 3 now and keep these episodes on a separate save slot for Diamond Rush, when diamond choices are free. For comparison, Dino's route in the same story is 297 diamonds, Lucifer's 223 and Andy's 154. The full list is on the Heaven's Secret page.