A Good Match?
But could it work in terms of their characters? Let's take a look at who we're dealing with here. On one hand we have Tia. She's domesticated. She has a simple job in a small town. She has an easy life overall. She doesn't seek fame or glory, just plenty of affection and attention. But she's no blushing maiden by any means. She's also bossy and pushy. She can be a bit reckless and that can land her in a few pickles (like in the Lake Cave when she tried to follow Maxim). But she's a resilient creature nevertheless. She was teased relentlessly as a child and lost the man she wanted to another woman, yet none of this broke her spirit. She's a tough cookie. In a relationship Tia offers stability and loyalty — as long as the man in question can swallow her strong personality.
On the other hand there's Dekar. He's a flirt, to be sure, but he's not heartless. He's a very caring soul who sacrificed himself for his friends. He has a way with words that, when properly implemented, can be quite charming. He seems to like some fiestiness in his relationships. He likes to argue with the people he's closest to, but he takes none of the jibes personally and isn't a vicious person. He's animated. He's not shy about what he wants. Despite his ego, he's a true team player. He's not ambitious; he believes he's already great at what he does. This gives him a sort of a laid-back quality.
Going off the summaries of their individual personalities, I think Dekar and Tia make a reasonable match. They would have a stable life together. Tia would be happy being a homemaker (in addition to working as a shopkeeper, or innkeeper, or whatever) while Dekar teaches Prince Alex swordsmanship. With the Sinistrals gone for the next hundred years, there wouldn't be any real reason for Dekar to travel and do battle, so Tia wouldn't worry for his safety. And training the prince would be an outlet for Dekar's warrior urges. Dekar would be ready to cheer up Tia if she were ever sad, and I could see Tia finding Dekar's brag amusing, giggling at all his assertions of being the strongest.
Not that there wouldn't be problems. Tia might be frustrated by Dekar's stupidity at times, and Dekar might chafe under Tia's bossiness. But I don't think either of those things would break them apart. These are two characters who desire companionship deep down, who aren't mean-spirited or selfish.
The next issue is one of practicality and reality: Could Dekar and Tia ever be together? Is it possible? If we look at the ending of Estpolis II, it would take some imagination to get that idea off the ground. Tia's back in Elcid mourning Maxim's death. Dekar's far away in Bound Kingdom. There aren't any apparent reasons for either character to travel to the hometown of the other, but maybe it doesn't have to be as extreme as that. Maybe they can meet up somewhere in between, like at Guy's party in Tanbel. Surely he'd invite Tia too; I doubt if he forgot about her. And if Tia brought all her emotional baggage there, Dekar would comfort her once again.
There's also Tia's emotional state at the end of Est II. She spent most of her life longing to be with Maxim. She invested a lot in him emotionally and it never paid off. She might have some walls up around her heart after that. She might not allow herself to fall in love so easily again. In this case Dekar would have his work cut out for him. He would have to pull out all the stops to win Tia over. But Dekar seems to like challenges, so this might just intrigue him.
There isn't much to say about these two in game canon. We don't know what Dekar and Tia really think of each other. When introduced to Tia in Bound, Dekar doesn't respond to her greeting. When Dekar saves everybody in the North Dungeon, Tia is the only party member who doesn't comment on the situation. In fact, until the famous scene in Parcelyte, these two have absolutely no interaction whatsoever. But that's when things get really serious. Another side to both characters is revealed in that conversation. Tia tells Dekar something about her past that no one but Maxim knew. There's an element of trust to that exchange. It could serve as a starting point.
After falling for someone as emotionally stupid as Maxim, a sensitive guy like Dekar could be a welcome change for Tia. And I think after a long day of sparring with the prince, Dekar would like a little sweetheart to come home to, along with dinner (however inedible) and a clean house. Tia would keep Dekar in line, Dekar would be responsive to her feelings and they'd both get the attention they crave. And think of all the cute little blue-haired children they'd have!