I'm currently halfway through my first draft of "Siren's Song" - my next SF novel. I just emerged from a chapter with a point of view switch to my secondary character (first book I've done any head-hopping in). Meet "Water". She's a little over five feet, and is very much an aquatic sort complete with fins. She's silicon-based and comprised of bio luminescent crystals that clue you into her mood. If her fins are rippling orange, best to keep your distance. Violet? If she was a cat, she'd be purring.
Water is a hoot as a character, which is why I decided on a few rare dips into her mind. She's literally lived with my main character (who is human named Scott Rellant) and tends to think like his species. Save for the instincts and racial memories, of course. And the fact that she hates humans in general. Water is very much screwed up in the head because of her forced association with Scott, but her inner demons won't throw off my readers because being human is what makes an alien character work.
Everyone has an origin story, mostly involving parents or something equivalent as is Water's case. She loves, even though the poor thing is a bit confused about the terminology. Anger, betrayal, and enlightenment are all part of her package, as is an eventual reconciliation. Human things. Aspects that will draw readers in to empathize with her, one of the base requirements for any good character. And that's the point I'm making here - being alien doesn't mean you can ignore the rules of good characterization. Not unless you're after a comic-book Bug Eyed Monster. As a fully filled out character, Water is in a position to advance the main character, and to become a sympathetic comparison the reader may gauge him against.
Not that Water's a paragon of virtue. As a proper character she must bear her share of flaws. Even if the reader won't be looking at her point of view that often, they will still need to see her with depth enough to react appropriately to the main character or the events unfolding around them. She also gets to advance the plot rather than simply being a prop or device. She can be snide, witty, or insanely dangerous. Very much a sidekick.
And she's a siren, but not the kind who lures sailors to their death. Hers is a more direct approach:
The lake reverberated with thuds, announcing the arrival of a new menace. Skimming the sandy bottom, Water saw a cone-shaped metal fish arrow toward her, its tail bright and hissing. Twisting around, she flattened her dorsal fins like crystalline wings and chirped. It took only a moment for the taste of the approaching projectile’s vibrations to give her the right pitch. Constricting her throat, Water screamed. The water in front of her vaporized into a narrow beam directed at her adversary. The projectile buckled and then shattered.
As a prologue can offer a unique perspective on both the main character and the story, Water gives the reader a glimpse of both from "the other side" of the equation. Her own struggle between friendship and duty is indirectly a shining light on the main character's own values and sensibilities. In Scott's case, he finds himself falling short in both categories, and finds in Water an unplanned for guide to his own redemption.
Aliens. Sometimes more human than us. And that's the way I like 'em.