Valcen is a Threadwielder. Despite most of the book not being from his point-of-view, he can be considered the primary protagonist of A Thread Between The Stars. His name stems from the word valcee (strength).
At heart, Valcen is a scientist. The physics of the universe he lives in fascinates him, both in what it is and what it is not. He's always dreamed of making his own universe - not necessarily a self-unpacking one like the one he was born in, but something grand and exciting and altogether different.
He's driven by curiosity and has a voracious hunger for knowledge. Occasionally, his projects, while furthering some other agenda, end up being excuses for this dabbling, such as his excursion into kavkem neurology (as he did to create Ne͡ivat'va and Gazhil), which undoubtedly is time he could have put to better use.
Much like Jeneth (or, indeed, any other Threadwielder not saturated in a mortal culture), he feels no particular empathy toward Evenatra, kavkema, Nayabaru or humans, though it may serve him at any one time to ally with them, and he may, through his avatar, feel genuine emotions aiding him with such a decision. Nonetheless, fundamentally, he doesn't care.
Valcen is not one of the original Threadwielders that the universe was seeded with. Instead, he came to existence approximately five hundred million years after the Big Bang, presumably through some kind of procreation.
An event simply referred to as 'Anigi' is described as deeply frustrating to Valcen, and a source of ongoing ostracism, exiling, and formal ban from using Thread. By the time A Thread Between The Stars begins, he had fundamentally rejected the ostracism and was stemming himself against it - most notably by hoarding Thread as well as daring (cautious) incursions into the territories of other Threadwielders.
It's implied that Valcen and Jeneth have enough of a personal history with each other that Valcen “harboured the hope that [Jeneth] had changed enough to be reasoned with”1) once he seeks out his help. It's also implied that Jeneth takes Valcen's ostracism seriously and feels that he deserves it.
Just prior to A Thread Between The Stars, he's spent several avatar generations on Sesakanala (a planet at the edge of Zalaagra's territory), which he would only cynically describe as 'cosy', but nonetheless considers an approximation of home.
Irritated and confused by the situation on Nekenalos as broadcast by Evenatra, but viewing it as an opportunity to benignly reintegrate himself into Threadwielder 'society' if he were to manage to solve the conundrum, Valcen investigates the planet. After talking to Evenatra, he tries to see if there's anything he can do about the weapon (Terenyira) directly, but flees before a direct encounter tests his resolve. On accident, he leaves Ne͡ivat'va behind. Rather than attempt an actual attack on his own, he leaves for Earth, hoping to recruit Jeneth for help.
Valcen arrives on Earth. Aware that can only convince Jeneth to listen to him if he shows sufficient respect for the local culture, he chooses to incarnate as a human and live through human culture to adulthood - a small time investment, but from his perspective a sound one. In 1978, he's born as the human Anton Lusk.
Deciding that thirty years are sufficient symbol of his respect (and that the round age “[suggests] a certain punctuality”), Valcen breaks off all pretence of a normal human life to seek out Jeneth. He doesn't bother with good-byes, be it to a close friend Janice or his boss Mark, even though both would be easy (given mobile phones already exist, and he owns one), simply sweeping those ties away as irrelevancies, one of many indicators that his alliance with human culture did not run very deep.
How many of this civilisation's ethical quirks had Jeneth absorbed? Could he hope to appeal to notions of heroism? Responsibility of kin? Nobility? Fairness? Any number of appeals might dissipate in the hollow, permissive Threadwielder framework instead. He would try all of them; not because he felt they should have moral impact, but because they might.
A Thread Between The Stars: Satellites
Nonetheless, it is around this time that he begins to appreciate time on mortal timespans, showing concern over the eighty potentially squandered years he's spent getting from Nekenalos to Earth and incarnating as a human, and the time it would still take to get back to Nekenalos.
Driven by his desire to have help on Nekenalos, Valcen traded a millennium of his freedom to Jeneth in exchange for Jeneth's companionship and help on his quest.
“Perhaps a millennium at my command, counted in Earth years, discounting any transit times. Add a year for each year invested in Nekenalos… counted in full from its first day. Give a few years as interest when pausing and resuming the payment at mutual discretion. Thread intervention is free, but I reserve the right to refuse to use any more Thread from my provisions at any time.”
A Thread Between The Stars: Satellites
When leaving Earth, he killed his avatar after swimming out into the ocean.