He's a marriage candidate for Elodie. His early appearance could be said to be the player's "introduction" to the courtship dynamics/conflict in the story, and depending on how Elodie responds may affect future events. See below under Involvement.
He seems to have vested interests in Nova...
Spoiler Warning: Plot details follow, read at your own risk
He will send a necklace to Elodie in the hopes of arranging a betrothal. Elodie can learn about this beforehand by having Foreign Intelligence > 10, and with a Court Manners > 10 check, she will understand the social implications of wearing a man's gift; failing this check causes Elodie to wear and display it, implying she has accepted his courtship.
If Elodie accepts his proposal, they will become engaged and she will be given the option to invite him to the gala on Week 17. This gives her the option of bypassing the Dance check and gives her different options to deal with an upset Banion if she has displeased him.
Because of the intermarriage between Sedna and Elath, if Adair is assassinated Elodie gets the option to make Talarist Duke of Elath, provided he gives up rulership of Sedna and pledges to Nova. A high Intrigue score discourages this because of the strife the intermarriage has caused historically.
Although Talarist himself is too young to have been directly involved in the conflicts of the past, he is aware of the connection between himself and Adair and there are implications he may be responsible for the assassination attempts. If Elodie is engaged to Talarist, Adair will not be attacked, but if Adair is killed and his lands given to someone else or betrothed to Elodie, Talarist will raise high tariffs or simply attack the new heir's lands.
In one of his marriage epilogues, it makes note that he encourages Elodie to distract herself while he enjoys his new position in Nova's nobility. In another, Elodie and Talarist raise two daughters and a number of exotic pets from the east. The King Consort never tired of giving his girls presents to make them smile. If Elodie is unable to have children, Talarist will request a divorce.
If Elodie rejects Talarist rudely, orders her agents to look out for foreign powers and you pass a high Trade check, you can cause Talarist's tariffs and border restrictions to backfire. If Elodie chooses to marry him at the end of the game, flavor text will directly imply that this was done to calm the border dispute.
Talarist can make a useful ally. If Elodie is engaged to him she can use the alliance with Talasse against Ixion in Week 10, and although he does not send troops when Shanjia invades, if the invasion is repelled he can help with the aftermath.