Quite often we just go back to my flat and do stuff together. In the past few weeks he's become keen on cooking, with varying results. His enthusiasms flare up and then they die away again. He went through a phase of playing games of patience. He would have to complete the game before he did anything else. If he managed to get it out, it was a good omen, but he hardly ever managed it. In the summer he was fanatical about jigsaw puzzles: he brought one to my flat that was called 'The World's Most Difficult Jigsaw'. It had thousands of tiny pieces with pictures on both sides. And you didn't know what the final image was meant to look like. For weeks, I couldn't use my table because bits were scattered over it, straight sides at one end and in the middle the gradually emerging picture of a street scene. Suddenly he became bored. 'What actually is the point of doing jigsaw puzzles?' he said to me. 'You work for hours and hours, and then when you complete it you break it up and put it back in the box.' He worked for hours and hours, but he never completed it and it's now in a box under my bed.

Where did it go wrong? That's what my mother says sometimes, especially when Troy is silent and withdrawn, skulking in his bedroom, his face a sullen mask. He was always clever, sometimes bafflingly, dizzyingly clever, talking at one, reading at three, dazzling teachers with his aptitude, shown off to my parents' friends, paraded in assemblies, showered with school prizes, written about in the local paper, put into classes with children who were one, two years older than him – and two feet taller than him as well because he never seemed to grow. He was tiny, with bony knees and sticking-out ears.

He was bullied. I don't just mean pushed around in the playground or jeered at for being a swot. He was systematically tormented by a group of boys and excluded by everyone else. The bullies called him 'Troy Boy', locked him in the school toilets, tied him to a tree behind the bike shed, threw his books in the mud and stamped on them, passed notes around the classroom about him being a sissy and a gay. They punched him in the stomach, ran after him at the end of the day. He never told anyone – and by this time Kerry and I were so much older than him that we occupied entirely different worlds. He didn't complain to the teachers or to my parents, who just knew that he was quiet and 'different' from the other boys in his class. He just worked harder than ever and acquired a pedantic and slightly sarcastic manner that of course isolated him further.

Finally, when he was thirteen, my parents were summoned to the school because he'd been discovered throwing firecrackers at boys in the playground. He was wild with rage, weeping and swearing at anyone who came near him, as if the results of eight years of abuse had surfaced all at once. He was suspended for a week, during which time he broke down and 'confessed' to Mum, who stormed round to the school making a fuss. Boys were hauled in front of the head, given detentions. But how can you tell children that they have to like someone and be their friend, particularly when that someone is like my little brother: shy, scared, socially dysfunctional, crippled by his own particular brand of intelligence? And how do you undo damage that's been built into the foundations? With houses, it's easier to pull the whole thing down and start again. You can't do that with people.

I had left college by this time. I didn't understand how serious it was until Troy did his GCSEs. Maybe I didn't want to understand. He was expected to do well. He said the exams had gone fine, but he was vague about them. It turned out he hadn't done a single one. He'd sat in the park near his school, throwing bread to the ducks, staring at the litter on the banks of the pond, looking at his watch. When my parents discovered this, they were stunned. I remember being with them one afternoon when all Mum did was cry and ask him what she'd done wrong, was she such a bad mother, and Troy just sat there, not talking, but on his face an expression of triumph and shame that terrified me. The counsellor said it was his cry for help. A few months later he said that Troy 's cutting himself – dozens of shallow abrasions across his forearms – was a cry for help. And the way he sometimes didn't get out of bed in the mornings – that was a cry for help too.

He didn't go back to school. There was a private tutor and more therapy. He goes three times a week to a woman with letters after her name to talk about his problems. Every so often I ask him what goes on in these forty-five-minute sessions, but he just grins and shrugs. 'Often I just sleep,' he says. 'I lie down on the couch and close my eyes and then suddenly there's a voice telling me my session is over.'

'How's it all going?' I asked as I made us a pot of tea and he cut red peppers into strips. Already the kitchen was a mess. Rice bubbled ferociously in a pan, making its lid bump and water splash over the sides. Eggshells littered the table. Bowls and spoons stacked up in the sink. There was flour on the lino, as if there had been a light snowfall.

'Have you noticed,' he asked, 'that people always ask me how I am, in that careful, tactful kind of voice?'

'Sorry,' I said.

'I'm bored to death with talking about me. How's it going with you?'

'OK.'

'No, you're supposed to really tell me. That's the deal. I tell you, you tell me.'

'Actually, "OK" is about the right word. There's nothing much to report.'

He nodded. 'Brendan's going to teach me to fish,' he said.

'I didn't know you liked fishing.'

'I don't. I've never done it. But he says one day we can go to the sea where a friend of his has this boat, and fish for mackerel. He says you just haul them out of the water, one after the other, and then cook them at once over a fire.'

'Sounds good.'

'He says even if it's raining, it's nice to sit in a boat waiting for a tug on the line.'

'Have you seen him much, then?'

'A couple of times.'

'And you like him?'

'Yes. Can't imagine you with him, though.'

'Why not?'

He shrugged. 'He's not your style.'

'What's my style?'

'You're more of a cat person than a dog person.'

'I don't have a clue what you're on about.'

'He's more like a dog than a cat, don't you reckon? Eager, wanting to be noticed. Cats are more independent and aloof

'Am I independent and aloof, then?'

'Not with me you're not. But with people who you don't know so well.'

'What are you, then?'

'An otter,' he said immediately.

'You've really thought about this.'

'And Mum's a kangaroo.'

'Kangaroo?!'

'And she can't quite get used to the fact we're no longer in her pouch. Except that I crawl in and out occasionally.'

'What's Dad?'

'Brendan once had a kind of breakdown as well,' said Troy. He started threading alternating chunks of lamb and pepper on to skewers.

'Did he? I didn't know that.'

'He said he never tells anyone. But he told me because he wanted me to know that pain can be like a curse and like a gift, and that it's possible to turn it into a gift.'

'He said that?'

'Yes. He's a bit of a hippy, really.'

'I'm going to have a beer, I think.'

'Dad's a duck.'

'I don't think he'd like that.'

'Ducks are all right. They're optimists.'

'And Kerry?'

'What about gazelle?'

'Has Brendan said anything to you about me?' I tried to keep my voice casual.

'He said he hurt you.'

'Ah.'

'Did he?'

'No.'

'And he said you were too proud to admit it.'