I'm going to do something I personally dislike, somewhere in my ramblings it will be there.
Alot of this seems to be that your the boring one, that you don't stand out from the companions because they have a backstory, they have goals, and aspirations, dreams or what not. The custom character doesn't, he's a blank slate. To me that blank slate means that I can write what I want, be who I want to be, I don't need to focus on backstory, I can and do. Yet it's not my backstory that is driving my character forward, so what is it? Is it this thing in my head going to erase me from existance? Is it something less pressing just a wander lust, a drive to adventure? The calling of my goddess? The whispers of the Great Old One within my head? The desire to seek out stronger and more powerful magic, a lust for battle?
How these questions impact the blank slate. I'll use 2 of my characters Keissa and Brulk
Skill checks Keissa always goes for persuasion, never intimidation. Brulk never tries to persuade unless it's hostile negotations. If it fails Keissa tries to knock out those that attack her, and heal them afterwards. Brulk gives no shits and just hammers them they were dumb enough to attack him, he warned them. From this we can see some of their values as a person Keissa would much prefer to talk non threateningly to people, and if she says the wrong thing (failed check) she still tries to refrain from killing people. Brulk is forceful he's right your wrong, and he'll not hesitate to settle it with his axe if need be. In this sense Keissa is unique to her companions, La'eazel is forceful quick to anger, and attack. Shadowheart is to me fickle. Ast is blood thirsty and hates talking any less it gets him something to me. Gale is all about knowledge he is relatively good but he cares more about knowing stuff. So Keissa is different then them. Brulk is for the most part more like La'ezeal then the others in his party.
Keissa won't hesitate to kill goblins, or other 'evil' people or Kahga for that matter she protects innocents and tries to better the world. Brulk doesn't care who he kills, they were in his way, little girl dies thats on her, she's not important. Whats important is getting tadpole out of his head, earning money, and smashing goblins, among other things.
So you see your blank slate isn't so blank anymore your pc begins to take shape, and form. Sure the others have great quests to undertake, for your pc every step is part of a new quest, a new discovery. Even owlbears are new to them, should he kill it? Try to help it, run away? Your pc isn't pregenerated, it's who ever what ever you want them to be. So your pc is taking shape as you play, sure their not pregenerated with set voice styles, or distinct patterns of speech. So that leaves them open, (which seems to be some of the problem) they can be who you want them to be, have quirks that you want them to be. maybe Brulk drinks every night at camp, hates vampires, but gets randy around githyanki. It's up for you to decide, for you to chose. Sure you got baggage with you but their only there to tell your story, serve as meat shields, mobile health potions, a verbal spar. They add to your story by how you chose to interact with them. Perhaps your original way of thinking about them changes, as you learn more about them? maybe that half-ear talking about torture makes you enjoy her more? Again up to you to decide.
RPG characters when not predesigned or preset can turn into the greatest of characters if you let them, and in the choices that are offered to you by Larian there is a good deal of flexibility for you to turn that blank slate into an adventure worthy of D&D. A story to tell your friends, to share with loved ones. Your fights no matter how they make you want to rage quit, may in time be something you laugh at, or if you shared it something you joke about or are teased about.
Everyone will find out the stories of Ast, La'eazel, Shadowheart, Gale, Wyll, but not everyone will know the travels of Keissa the Mouse, or Brulk Bloodspray, Dinglehemer, Slayer, Frost Wyvernfoot, Thruik Axefeast, George Paul Rameriz Vedictus III. Why is that because noone else will do every thing the same as you, so your blank slate is just a starting point to creating something unique, that you can claim as your own, make your own. Make it not about whoever designed the preset characters, but make it about you.
NO Backstory = there are many games that do it, some do it to often, to blandly. Skyrim comes to mind with limited dailogue, far to much combat, and limited interactions between NPCs. Sure it's a great hack and slash rpg (I use the term rpg loosely in this case. Look at the story though, look at the choices it offers to you. Can you choose to side with Alduin? Can you Choose no side in the fight for Skyrim, and still complete the main quest? Can you choose to kill Jarl Ulfric when you want to, or any of the Jarls for that matter? How about joining the bandits? Most of these are no's, some you might be able to. Yet in the long wrong your given the false sense of freedom of choice. Because the game itself by design limits what you can do to the world around you.
Here larian is offering you choices, upon choices. Do you simply attack the druids, with no conversation, do you try intimidation, or persusion? Do you try to sneak past the druidic guards, and assassinate Kagha? Do you ignore what happens to the child, or take your revenge, or do you do something in the middle? Do you free Us and cripple it? Do you free Us and travel with him (take him to the helm, unsure if he shows back up. He'd better.) Do you free him, and simply leave? These are the choices that begin your story, that begin to shape your PC. That turns it into your PC, each of these actions have consequences. Does your PC care about the consequences good or bad? Do they stop and reflect try to see every possible consequence? Do they side with someone just because of their race, or class. Do they hate their own kind, so are more then likely to go against the wishes of someone of their own race?
I know this isn't what some want, that this may be to much thought into your blank slate. That all these questions are just to much, are maybe not enough. That in some way even if you answer all these questions, that you still feel overshadowed by your companions. I get it, is it me trying to play devils advocate which I do alot, No. It's me as a writer, as a creative person one that loaths being forced into a pregen'd character, that despises the false sense of freedom of choice that many games force upon you. I personally feel that Larian is offering a wide variety of choices, sometimes more then others. That they are seeking to offer Us the players enough choices that we develop characters that we love, and cherish. That we want to tell our friends about, that will make us rage at the game for hurtin, that will make us cry because we had to reload, that will make us foam at the mouth because we had to go against what we want by the actions of an NPC.
You see a blank slate, and think it will be a blank slate forever. I see a blank slate and want to fill it up. Thats the heart of any true RPG, you start with a blank slate (your character) you begin to fill that slate up by giving it a sex, assigning attribute points, skills, abilities, hair color, eye color, a voice (really need more options and different ones for dwarf, halflings, and githyankis. (Atm IMO the voices only work for the other races, albiet some are a stretch. Yet they don't work for dwarves who's voices IMO are far deeper, halflings more childlike with some depth. and githyanki's rougher more volatile.) a class, a race. Your slate to me fills up more on how you deal with the companions upon meeting them, and continues to fill up with each thing you do, or fail at. Did you nearly get a tentacle hug from the Mind Flayer, did you kill the enthralled fisherman with no regard, and no conversation?