You've gotta hand it to Euan MacIntyre: he may be one of Possession's very minor characters, but his charm, insight, and daring really steal the show in the last few chapters of the novel. And that's not all: it's thanks to him that the last few stages of the novel's plot come together in a neat little bow.
Euan enters the story briefly, in Chapter 7, as one of Val's temporary employers. He's a solicitor (that's British for lawyer) at a London firm, and Val has been doing some typing for him. When Roland Mitchell first meets him, Possession's narrator gives us little tidbit of insight into Roland's perception of him: "There was something powerful about him, Pluto delivering Persephone at the gate of the underworld" (7.86).
Uncharacteristically, Roland has his analogy backwards. In Greco-Roman mythology, Pluto (Hades) abducts Proserpine (Persephone) and carries her to his underworld kingdom. When he "delivers" her, he brings her back to the bright, sunny surface of the earth. In Possession, Euan does things the opposite way around: in the end, he's the one who rescues Val from the dank, gloomy "underworld" of her basement apartment, and from her rotten relationship with Roland, too.
As Possession's narrator tells us later, Euan "had always loved mending things. Broken models, stray kittens, grounded kites" (22.58). That love is at least part of what attracts him to the bitter and heartbroken Val, and his cheerful personality and kind attention quickly help her to transform from a being a resentful, passive-aggressive cave troll to being a glowing, happy young woman.
Euan's signature skills benefit Possession's other characters, too. He's the one who first figures out that Maud Bailey may own the copyright to the Ash-LaMotte correspondence, and he's the only character who suspects—long before the discovery of Christabel LaMotte's final letter to Randolph Henry Ash—that Maia Thomasine Bailey may have been Christabel's daughter, not her niece. Talk about being a useful guy to have around.