Two minds, one body
In many ways, Tom made life more bearable for Harry. Sure, he was a bodyless voice providing commentary while living in his head, but it was a step up from having no friends at all. At least Dudley couldn't drive him away, like he always did with real people.
Tom always comforted him whenever aunt Petunia yelled at him, he always consoled Harry whenever uncle Vernon took the belt, and he always helped Harry deal with Dudley's bullying and beating. Harry's body always healed quick after those, sometimes overnight, with only faint scars remaining. It had started after Tom had begun talking to him, and Harry liked to joke that Tom kept him alive.
Tom didn't find that funny in the slightest.
Harry also soon found that Tom liked to give advice on how to avoid Dudley and his gang. With his help, he managed to find the best hiding spots around St. Grogory's, in the hopes of avoiding them, though that didn't always work, and he still had to resume running away from them, lest he be the victim of yet another round of 'Harry Hunting'. Of course, that usually led to a whole load of different problems, like that one time he had somehow found himself on the roof of the school's kitchens while on the run. Neither the headmistress nor the Dursleys were really amused by that, and didn't quite believe that Harry also had no idea as to how he had gotten up there.
He'd been locked in the cupboard for several days after that, with no food or water. At least he had some company in Tom, which made it kinda bearable.
Though, as Harry would eventually discover, Tom was no angel himself. His backhead voice had a vicious streak that rivaled Dudley's, even if he was nowhere near as overt in it. Whenever Harry was 'wronged' by anyone other than the Dursleys, bad things tended to happen to them, far too often for Harry to really think it coincidental.
After Mrs. Joanne once sent him home with a note after her wig had somehow turned blue - something for which she blamed Harry, even if he had no idea how (not that that mattered) - she didn't come back to school the next day, and he later found out that she had been hit on the head by a brick. When Dennis once threw a rock at Harry's head (it missed, fortunately enough, Harry did not think even Tom could've healed that), he later fell from a roundabout and cracked his skull, turning him into a vegetable. After 'Aunt' Marge once had her mutt Ripper chase Harry up a tree, forcing him to spend the night outside in that tree, with the bulldog barking at him viciously, her car was rammed by a lorry on the way home, breaking multiple of her bones and nearly killing Ripper. (She didn't come back the year after, for which Harry was thankful.)
Tom had laughed himself silly at each of these accidents, calling it 'only fair' that they suffer. Harry didn't know if he agreed; on the one hand, he didn't like people be hurt, but on the other, with how they treated him... he did have a hard time feeling sorry at times, admittedly.
And even if Tom could be nasty, he was also helpful. Harry had learned the hard way that being better than Dudley was a fast way of being punished, and so he had let his grades drop quite a bit (though he still somehow managed to eclipse his cousin, which said quite a bit about Dudley's lack of wits), but with Tom's help, he still managed to at least understand the various subjects even without passing the tests. His maybe-imaginary friend was rather brilliant, all things considered, able to explain quite a few things - especially mathematics - to Harry in a way that he kinda-sorta understood (even if Harry wasn't quite certain what Cosmic Geometry or something he called 'Arithmancy' was or for what he'd need it).
Really, the only thing about Tom that did somewhat irked him (other than that vicious streak), was the fact that Harry honestly knew very little about his friend. Truth be told, Tom himself seemed not sure about who he was, either, which Harry did believe to a degree - though he also still believed that Tom was little more than a figment of his imagination, a fictional character he had come up with to deal with his own loneliness.
Still, there were things he could not quite explain, flashes of what seemed like memories in his dreams, making them nightmares. He suspected some of these were of the car crash which had killed his parents, but there were other things, images he couldn't place, which he honestly suspected belonged to Tom. The voice in his head was of little help there, however; while Tom did know of these apparent memories, he also struggled with placing them in any sort of context. In many ways, the strange folks who sometimes came up to Harry for some reason or another - like that weird guy who had bowed before him at a bookstore once - actually helped in that regard, because something about them managed to stir something from Tom's mind.
That way, by the time roughly four years had passed since Tom had first made his presence known, the two of them had at least a rough idea as to who Tom was. His name had apparently been Tom Marvolo Riddle (what a weird name, Harry had thought), and he had grown up in an orphanage in London during the twenties and thirties before going off to a boarding school in Scotland at age eleven. Apparently, he had also been the target of bullying, before putting that nasty side of his to work and essentially becoming a bully himself (Harry did not quite know how to react to that, so he didn't), something which may have ceased when he went to said boarding school. However, that was roughly where the memories ended; Tom sometimes spoke a bit of the holidays, when he had to go back to the orphanage, and of the horrors of the Blitz, of the bombers which flew overhead, of the bombs that fell, of the sound of the defense guns, and of the fires which would ravage London. 'I was afraid then', he admitted once. 'For my life, for my health... I wondered if there were ways to avoid certain death.'
'For supposedly the most civilized species on this world, these muggles are distustingly savage', he'd also say.
That was another odd thing about Tom, Harry found: he sometimes used words which made very little sense to his ears. The Dursleys were 'the worst example of muggle degeneracy', Mrs. Figg - the elderly cat lady who sometimes watched Harry whenever the Dursleys went off to do something nice - was a 'pathetic squib', and aunt Petunia's herbal remedies which she sometimes used instead of normal medecine were 'a fool's attempt at potioneering'. He would sometimes joke that 'the old walrus would have to use a love potion to enchant any decent woman, with that look of his' or that 'that insufferable mare of a woman ought to be a Legilimens, with her aptitude to discover secrets'. When prompted by Harry, he would explain that he kenw these words from his proper life, before being stuck in Harry's mind, though he always had to sheepishly add that he had no idea where these words had come from.
Harry wasn't sure he fully believed that last claim, but he also didn't needle Tom over it for fear of alienating his only friend. And as Harry would eventually find out shortly before his tenth birthday, he could actually peek into Tom's mind and see his thoughts, just as he suspected Tom could do to him. What he found was scary; it seemed as though Tom's memories were shattered beyond any sort of repair, and that he had only managed to put together a small fraction of them over the past few years.
Harry vowed then and there to help his friend reclaim his life - and maybe his body, if possible. After all, Tom had, for all his faults, proven himself to be a friend to Harry, and it was only fair that Harry return the favour, or at least try to do so.
In that sense, the disaster of Dudley's eleventh birthday was actually a blessing.