So far, I've gotten a Musa to 19 and a Striker to 25.
Right now, I really want to try to get the Striker leveled while there's all the bonuses to doing so.
As has been mentioned, so long as you're fighting stuff in the level brackets they're suggested, most mobs just fall over to clicking them to death, and using other moves when mobs surround you.
What's been really nice is that when you log in and get all of the daily log in rewards, you generally can stack it to get where you'll get the extra xp bonus for being on the newbie servers, plus another 100% because you logged in, plus another 100% for an hour or 200% for a half hour.
Moving along the Spirit's storyline chain and moving to the next area (new mobs that you're at the low end of the range for), you can fire off all those buffs, and at least in the twenties, gain 4+ levels in that half hour.
I could see myself adding on more new characters, because with the way the Family system works, you have energy, cooperation points, and knowledge points shared between them, and I've been noticing that quests that knowledge point based ones (along with NPC amity) are also shared between characters, meaning there are some questions you only need to complete once, and after having done so, you just talk to the NPC on the new character and collect the reward for the quest immediately.
Thanks to the shared storage and houses in towns, I think later in the game I may just start tossing extra characters out there to have them sit in various far-flung places on the map and rather than traveling across the world with one character, I may just change to another just to save the time.
I only bought the base game, and I don't feel like I'm missing out on a ton, but I could see how having a pet and not having to loot the enemies would be a nice quality of life upgrade.
As a side note: If you're playing this game, make sure to go into the loyalty store and buy and extra character slot right now. It's a 1 loyalty reward, and you're given 100 loyalty a day. (normally it's something like 5000 loyalty).
(yet another bonus for the Striker release event.)