I'd say the captain of the ship exploring the Badlands. They usually have to make tough decisions in a dangerous and unpredictable environment, which makes them really interesting. Their leadership skills are put to the test all the time.
Captain Kirk is definitely one of the most interesting characters. His leadership and decision - making skills are always on display. Spock is also fascinating with his logical yet sometimes conflicted nature.
Picard himself is very interesting. His character has evolved over the years and in this story, we see a more complex and somewhat jaded version of him. Seven of Nine is also fascinating. She has grown a great deal since her days on Voyager and brings a unique perspective. Then there's Soji, a new character. Her journey of self - discovery and her role in the overall plot make her a really interesting addition to the Star Trek universe in this story.
Sunny is quite interesting. He has great combat skills and a complex past that drives his actions. His journey from being a cold - hearted killer to someone with a bit of a moral compass makes him stand out.
In the 'Star Trek Badlands Novel', I think friendship and cooperation could be themes. Characters would need to rely on each other in the difficult Badlands environment. They may come from different backgrounds and species, but in order to face the perils of the Badlands, they have to form bonds of friendship and cooperate. This also ties in with the overarching Star Trek value of unity among diverse beings.
Sisko is really interesting. He's a complex character, being a single father and also a Starfleet officer in charge of Deep Space Nine. His relationship with his son adds a human touch to his role as a leader. Then there's Quark, the Ferengi bartender. His constant scheming for profit and his unique view of the universe make him stand out.
Kirk is definitely one of the most popular. His adventurous and maverick nature makes him a great character to write about. He can be put in all sorts of situations in fanfiction. Spock is also very popular. His logical yet conflicted personality gives a lot of room for exploration in fanfic. Writers can delve into his inner turmoil between his Vulcan and human sides.
Q is a character that is frequently used in Star Trek crossover fanfiction. His omnipotent and mischievous nature can stir up all kinds of chaos when he interacts with different Star Trek crews. He can easily create situations that force characters from different series to work together or face challenges they never expected. Another popular character is Seven of Nine from Voyager. Her Borg - related backstory and her journey towards humanity can be a great addition to crossovers, especially when paired with characters who have different views on artificial intelligence.