WMG / The Exiles

Josselin has the other half of the Captal's Bequest

Caitlet is an incomplete Captal - Lusath Adennos died before he could pass on the full Bequest, which Cait spends decades of her life hiding (alongside her tremendous physical and mental damage thanks to Glenin). The Captal's Bequest is vitally important (although we don't know why), enough that any other Mage Guardian would happily die, or kill, to preserve the life of the Captal and the security of the Bequest. But right as the Captal died, Cait almost consumed the soul of a baby about to be born - as though the part of the Bequest she had was drawing the rest of it that the powerfully Mageborn infant had accidentally claimed, an act that was cut short by a quick Ladder trip. Josselin is that child... now all grown up and desperately in love with Cait, and the Bequest will only be whole (for its inevitable dire use) when Cait can let go of the pain and anquish Glenin poured into her and love Josselin in return - letting him in past all her defenses so she can draw on both halves of the Bequest.