I really like how intrinsically connected the scenes are to the gameplay and combat, the mystery of the scenes is what completes the experience. Because the male protagonist is majorly left in the dark about the blooming relationship between Aster and the Mentor, hints of their lewd acts are what will further improve the eroticism of this game and will become the defining feature that will make it stand out.
Consider the following:
1. If turn-based traps, as suggested by UniversalConquistador, are implemented, they could create opportunities for Aster and the Mentor to advance further down a convergent path while the player is delayed. The longer the player remains trapped, the more explicit their actions could become. When the player finally catches up to the scene of their encounter, descriptive details—like 'a strange, musty smell'—and tangible remnants such as Aster’s equipment or underwear, stained with a suspicious white liquid, could heighten the tension and narrative impact.
2. When the three are together and are defeated in combat, the aftermath could include glimpses of Aster's underwear through rips in her clothing from the battle. As her relationship with the Mentor deepens, her choice of underwear could gradually become more mature and provocative, reflecting the shift in their dynamic. It would be interesting to highlight Aster’s changing attitude with each attire upgrade—perhaps she becomes more flustered or evasive in her attempts to explain her predicament.
3. Camping scenes for extra-long dungeons, where the level of Aster's corruption will affect whatever happens during that rest period. Imagine. Maybe the protagonist needed to drink some water during this rest period but notices Aster's tent to be slightly shaking with muffled voices. Maybe when you try to investigate, only her head pops out looking out of breath and red on the cheeks. Maybe, after this rest period, Aster looks exhausted and seems to be clingy towards your mentor. She probably is just using him as a means to help herself stand...or maybe it's something else??? *wink wink*
4. When in town, the player has the option to train to improve their stats, such as HP, MP, and damage dealt, making dungeon diving a bit easier. At the same time, Aster will focus on training her Mystic Arts. The player is given the choice to train alongside her or pursue their own training. However, each time the player chooses not to join her, Aster spends that training session with the Mentor instead. Who knows what those two will do...
5. At certain affection/corruption thresholds, Aster and the Mentor might initiate their own dates, leaving the player with no choice but to spend the time alone. Maybe we could find something later down the line while exploring?
6. Aster x Mentor = THE Perfect Pair???
In any case, the creative foreshadowing of the events and the choices that are presented to the player are what I absolutely look forward to.