Yeah, you'll want play with other styles of a character as much as you can. Each style has unique modifiers that raise the chance of getting certain stats more often with rank ups, so it's a good way to round out their stats by leveling them since stat gains are shared between styles. Higher ranks have better stats overall, but usually have poor modifiers. Their real strength besides being a champ right out the gate, is their access to better skills though even A and S rank classes can learn some useful skills. Make sure you check the skill information to see which ones have better damage grades.
I finally got the Final Emperor in a pull this night, with his New Years variant, which is really nice. I just wish I could get one of the other Gustave pulls.
My main team is
SS Gerad [a Successors Resolve] His main strength besides strong endurance and finally getting an S rank move in the form of Vacuum Slash. He has an auto-abilitiy that gives him a defensive counter that reduces damage he takes before countering. Another ability that raises the damage of his counter moves, and another move that gives him the taunt status to take aggro. All of which are great moves for a front rank fighter. I also have a few styles for him which has helped me build him into a better all rounder, and helped me get a few choice skills.
S Gustave XIII [Gustave the Steel] Despite never acquiring a secondary style to boost his stats, this style has great modifiers with strength, endurance, agility, and dexterity all have bonus modifiers. His skills are pretty great too with the A rank Bull Crush and the group hitting Wheel Swing. His best attributes is his high health combined with both his ability to cover weak allies, and to survive a killing blow with 1hp. His third skill allows him to restore a 150hp at the start of the round with a decent success rate. I may lack his top tier SS rank style, but this style is no slouch.
SS Ellen [You Stay Quiet!] Ellen is my main DPS on this team with one of the rare SSS ranks skills Dynamic Chop which can do high damage even against enemies resistant to slash damage. She is also very popular as a character, so I've managed to gain a few other styles with her that also have some great skills like a group hitting attack and an SS rank move as well from her Valentines Day style. Two of her abilities greatly increase her damage and her third skill raises her crit rate, so even with her two other crummy skills, she manages to deal some serious damage.
SS Sophia [May My Prayers Reach You] My main healer and both tanky and a great arsenal of hammer moves. She's a healing machine with three auto-abilities that make her one of the best healers in the game for me. One raises her Love stat when she lands a regular move that helps with healing magic, another raises the effectiveness of her healing spells which is how a D rank spell like Healing Light can heal more than a better healing spell like Water of Life. Her third skill has her heal the whole party by a large amount if she is knocked out in battle, which has saved my hide a number of time. I've also acquired a few other styles for her, so I have access to some better hammer skills like Seismic strike, but more importantly, I have access to some useful offensive magic like Light Ball and Wildfire. Red Mage style classes are a bit wonky in this game, but she's definitely one of the good ones.
SS Rocbouquet [Watch Over Me Brother] Another fan favorite character, and as far as mage characters go, she's pretty brutal with her skill set. While her stat boosts could be better with only a single boost towards INT, her auto abilities include a move to increase her normal attack damage which is okay, physical strength isn't her strong point, but it does help. She has a move that increases her INT when she lands a blow which is great for buffing her in the first few rounds of a fight before a tough boss battle. Her final skill allows her to randomly charm and enemy at the start of a round, effectively giving you a sixth member and reducing the enemy by one. It takes forever to go away as well. Her spells are interesting because she's a rare dual elemental mage with a focus on Water and Dark Magic. This style comes with Water of Life, giving me a second healer on the team. Righteous Thunder, which is a very powerful group hitting lightning attack that gets boosted with water affinity weapons. Finally she has a good group hitting dark spell called Energy Storm. Though the main thing I've given her is Dark Wash, which is a powerful S ranks dark spell for single targets, and great for bosses. My only beef with her is that her dual elemental affinity means I have to choose one to boost, which is usually water.
My formation is Speculation, which boosts the STR and AGI of the front three fighters (Gerad, Gustave, and Ellen) while reducing the AGI of the back two members (Sophia and Rocbouquet). The formation enforces my love of blitzkrieg style tactics in battle. I'm also running a Christmas themed party with several of the themed styles from the recent event. I also have a SaGa Frontier 1 and 2 team, and I'm thinking of making a RS2 team as well.
I advise you to try your hand at the Tower and Attack Events. Both will reward you with a free character, including all Seven Heroes from RS2 and several fan favorite characters like Claudia, Asellus, and Coppelia. While the attack events are straight up boss battles, the Tower ones have some story elements to them. You should also do the shrine event since they play into the main plot a little. Though I should warn you that they become very difficult the further in.
Another thing to do is to replay older missions on higher difficulties. On Hard Mode, you can acquire S rank weapons and gear, and these items will often have a random bonus to a specific stat, making them great gear for going after the top tier stuff. There are also a few SS characters you can acquire as extremely rare drops from a few missions as well such as Rouge, Fake Robin, and one of the Lizardfolk. The other thing to gain here is items used for decreasing ability costs in the Dojo.
You should also do Zweig Coliseum and Spiral Tower Challenges whenever they come up. Zweig will get you tokens to acquire some rare S ranks weapons of every type while the Spiral Tower awards you with tickets you can use for the Spiral Character Summoning. The best part of these, is that as long as you replay them when they reappear to as high as you can, when the event reappears, it will only make you have to redo a handful of round when you start up again, but still reward you for the earlier rounds. the Spiral tower tends to reward me twenty summoning tickets and a ton of gems and gold every time it comes back. Likewise the Zweig event will reward you almost half of the tokens needed to purchase one of the weapons in the exchange, and you'll easily get the other half replaying the few battles you have to do.