My mc is a paladin who has a fairly serious personality. He had to become the father figure of his siblings at a fairly young age, after most of his family was killed. Him being a paladin, he obviously has a lot of involvement with the church. He's also the leader of a band of adventurers. I'm fairly new to writing stories, but I just wanted to know how to at least make a serious character like him interesting, because I never received any feedback on whether he was at least interesting or not, or even if he was liked. He has other traits like being open about his emotions and being able to appreciate other people's humor during downtime, but seriousness is one of his main traits. And I want to make it so that he can stand on his own in this regard, rather than having another character that would balance him. So I must ask, how do I make him likeable.
The same way you make any other character interesting and likable. Flesh him out and make him feel like a real person. Understand his motivations and goals, and let the reader inside his head (assuming he is the POV character). If he is the POV character, the reader should be aware when something affects him on an emotional level, for better or worse, even if he is hiding it from the characters around him. Know his story, and let the reader slowly learn it. He has things that are personal and emotional to him, and things that don't bother him as much. He has things he likes, things he dislikes. He has his own habits and his own perspective through which he views the world. As you said, his trait of being "serious" is just PART of his personality. You're already on the right track in understanding that it shouldn't be his entire identity. Just like a character who is a jokester should be more than just a walking punchline in order to be truly likeable, a serious character should be more than just a blasé stone wall, even if these things are the first impression they provide.
If your MC doesn't often react emotionally externally to things, or engage in lighthearted/playful behavior, then those moments where he does deviate from this norm are going to have weight behind them for both the reader and the other characters who know him, and such moments when a character is pushed past their normal baseline and how they react can be, for any character, a factor in how likeable or unlikable they end up being.
Yes, if all your MC does the entire book is be straight faced and refuse to engage emotionally with any situation or give a little to any other character, then they WILL come across as being pretty unlikable and two-dimensional. It sounds like you aren't planning on doing this, so I'd say that you're already on the right track as far as making him feel like a more real person. Keep at it. Ultimately if you do a good job building his character, then it's all going to come down to what KIND of person he is.
TL;DR Make him a well rounded person and his motivations, actions, and desires will speak for themselves when your readers form their opinions on him. A character doesn't have to be obnoxious or flamboyant to be likeable. There are many things that make a character likable. Being interesting, relatable, perceived as "good", are all things that help. And these things can take many different forms.
Stories borrow heavily from morality roles; we have a hero that is acting pretty selflessly, and a villain that is acting pretty selfishly.
In Die Hard, Bruce Willis could at any time walk away, but he is determined to risk his life, despite a litany of injuries and life-threatening acts, to save his wife and child, even though they are separated or divorced or whatever. He is the hero, sacrificing himself for the betterment of others. The villain acts out of pure selfishness, greed, killing others without compunction to get his own way.
In the "Elementary" TV series, Sherlock is nearly always extremely serious, that is his default mode. A serious investigator, a serious student, constantly practicing his escapes and understanding of crime. Yet he is likable, because we see him risk life and limb to bring murderers to justice. He gets physically beaten, emotionally injured. About the only time we see him happy is when he finds the epiphany that solves a case. But we get 15 seconds of a smile and a toast, then he is off to be serious again.
Along the way, we see Sherlock make mistakes by being TOO serious. He ignores somebody's feelings, emotionally injures them, but sincerely apologizes and aims to make amends.
He shows open disdain for Watson's attempts to find a missing homeless man, but when he realizes she has a personal reason to do so, and then realizes her attempts uncover a kind of slavery ring, he is chastened, apologizes, and then with her gathers many of his old blankets, and the show closes with them going to distribute these to the homeless.
Sherlock is often, though always serious, a caring friend, finding and delivering aid to those in need.
Don't make your serious character perfect. Readers like characters that are trying to be good people, to help others, to improve society. People that show some significant degree of selflessness. They spend their time, and/or money, and/or talents doing something worthwhile for others.
For Sherlock (in "Elementary"), obsessed with crime, that is law enforcement that nobody else CAN do; he has devoted his life to society in this way, bringing killers to justice. The audience likes Sherlock for that. Even if Sherlock is abrasive, we think our town, our city, our world would be a better place with Sherlock in it.
Villains are anti-social, they damage society by pursuing their own selfish interests such as wealth, power or just gratification by sex or cruelty or murder. Those are the animals that Sherlock hunts. We see Villains hurt people for selfish reasons, physically, emotionally or financially. We don't like them, they would not be good people to have in our lives, in our communities, in our world at all.
You make your relentlessly serious character likable by giving them a healthy dose of selflessness. They don't need a sense of humor, they don't have to "relax" or be a good person to have a beer with. They can be emotional. They need to be trying to help others.
Invent the scenes where your serious character has those opportunities to serve the community.
I saw a story with a crotchety old man in his sixties. He's on the way to town for supplies, and sees a fawn stuck in a mud hole, struggling, its mother watching. He stops, wades into the mud hole and rescues the thrashing fawn, getting beat up, falling in the mud, but he persists and manages to embrace it and pull it from the mud. The baby is saved, and trots away with its mother. The thoroughly muddy old man gets back in his truck, drives to town for his supplies, the clerk says, "Whoa, Jake, what happened to you?" Jake says, "Oh, this and that. Slipped and fell in a mud hole."
And that is a serious but likable character. He sacrificed for others without any thanks or reward, he saved a baby, and took no credit. We like Jake from the start, he has proven his character.
Give them a character arc, development, impact of plot and depth. Characters are often likeable if handled correctly. Since your mc is a paladin with a serious nature, explain why they are serious; give them depth. explore their character, inside. Does he have redeeming qualities? Because that is another factor of making a character likeable.
Show the different sides of him; his emotions; his interactions and relationships with his band as a leader. Though, do give him flaws as it makes him human. The readers must also understand him, in his POV. Give him motivation and goals so that the readers can actually understand and root for him in the story. His world view, family dynamics, relationships, thought process, etc. As the plot progress, he must also develop. Show a change on how he sees the world or his personality change. Its important to show the more emotional side of him that makes him more than just 2 dimensional character. His actions that push the plot also matters as the mc. Like motivations and goals, the readers must root for him.