Lina Soh is the human Supreme Chancellor of the Galactic Republic during the High Republic era, in office from 234 BBY. Charles Soule created her; she was first mentioned in his short story "Starlight: Part One: Go Together" in Star Wars Insider 199 (15 December 2020) and debuted properly in Light of the Jedi weeks later.
The Great Works
Soh governs by programme. Her administration builds the Starlight Beacon station, runs the Republic Fair, and pushes an expansion of Republic membership into the Outer Rim underwritten by a slogan about unity and a matched pair of targons that follow her everywhere. She asks the Jedi to plant frontier outposts to match. It is soft power on an industrial scale, and for a while it works.
The problem the era is actually about
Everything the Nihil do is calculated to make the Chancellor's optimism look like negligence, and by The Fallen Star it does. Soh is not written as a fool — she is written as a competent politician whose entire theory of government assumes an adversary who wants something. Marchion Ro does not.
Why she is worth the space
Because the prequel films made the Republic's decline a matter of procedural corruption, and the High Republic makes it a matter of a good government meeting a threat outside its imagination. Soh is the Chancellor the era needs in order to make that argument, and the books do not let her off.