TW: Implied Kidnapping, Implied Confinement, Implied Death, Toxic Mindsets.
A.N. - I had such a fun time writing this! This is them together. If you want separate headcanons, let me know.
Platonic:
Pinky is a firm believer in unconditional trust. The notion that their friend is lying or manipulating him is alien, and he reacts with shock and disbelief whenever Brain suggests otherwise. He will proclaim their innocence as they walk out the door, denying Brain's assertions that he has been duped and impeding the genius's attempts at thwarting their escape.
The air is filled with Pinky's rambling at all hours of the day. He subjects his friend to a plethora of disjointed thoughts and existential questions that stem from the simplest of actions, such as the taste of a piece of cheese or a bizarre dream of being a robotic clone of himself. The mouse bounces from topic to topic without respite, believing his eternal conversation to be the ultimate form of admiration.
Upon nightfall, the Brain drafts plans for world domination. Every schematic diagram of destruction includes a designated safe house for his friend in the event of mass human interference. The genius delights in illustrating his agenda with dramatic monologues, drawing satisfaction from each compliment bestowed by his friend. Their criticism is the only critique that obtains a modicum of reflection, and the inevitable failure bolsters a desire to prove his intellect.
Romantic:
The Brain retains a cold and intolerant demeanour around their partner, a fact that draws many objections from the genial Pinky. The eccentric mouse encourages Brain to express a friendlier side of himself and implores him to apologize every time he upsets them. Brain denounces this as coddling and worries that it makes their partner impudent, but if they convey an aversion to him, he suffers a twinge of guilt.
Be it a malicious A.I. that eradicates their enemies or an endless supply of their favourite meal, Brain seeks to earn their partner's favour via sporadic gifts. Pinky handles the cuisine through a combination of dumpster diving and nighttime raids on the nearest grocery store, while Brain ransacks the laboratory and pilfers spare parts from the pockets of scientists.
The mice eliminate all ambiguity with their overbearing attitude. Pinky is overflowing with affection that he sees no need to repress, and Brain is convinced that their partner should shower him in gratitude for inviting them into his inner circle. Despite opposing Pinky's reckless passion, the genius incorporates a sense of urgency into his missions when their partner is brooding or distressed. A collection of trinkets and trophies are presented to them, and any souls that perish in the process are dismissed as collateral damage.