Write about someone who has a facade opposite to his/her true being.
Mad-Eye Moody was exactly that: moody with a mad eye.
He was in charge of a lot of people in the middle of a war and he watched them injure themselves, die or quit because it was all too much. He became gruff and rude, and acted as if everything and everyone was troublesome to him, like they were a chore. He still trained them, made them aware that life wasn't easy but he still lost them.
James Potter and Sirius Black. They were the two losses that struck him the most. Two young lads who didn't think even a fly could hurt them and that they were invincible; he trained them like any other and grew attached – despite his rule against it – and watched them come back from battle after battle. Seeing them in the Order made him think higher of them (though he never mentioned that to anyone).
The war progressed and James goes into hiding, having to leave his Auror days behind and Alastor – for he doesn't have his eye yet – feels a twinge in his heart when he sees the empty desk for the following few days until he gets replaced. Then the war ends, Sirius is arrested and James is dead.
No more attachments.
He grows colder, considerably colder; tougher and always striving for perfection. People flitter through his life but he pays them barely any attention because he's not allowed any attachments. Years go and the war returns, it doesn't reach a full scale battle but the rumours of his return are there.
Harry Potter. Just like his dad and his heart – the heart that hadn't twinged in over fourteen years – twinges more than he'd ever imagined. The resemblance is uncanny, younger than the lad he'd trained but undeniably the same. His friends, Hermione Granger and Ron Weasley, remind him of the friendship James had with Sirius and the heart twinges again for the loss of that young lad too.
He keeps up his cool exterior, not caring and acting gruff but he cares. His old and cold heart falls for the young trio and he is filled with affection, caring about them like they were his own. He never tells anyone though, training them when in the Order to work harder and making them the best that they can be – it's his way of showing he cares.
Everybody believes that he's gruff and unloving, a man filled with so much sorrow and hatred in his lifetime that he is incapable of caring. Nobody ever finds out that he has experienced so much sorrow, enough to turn him away from caring, but still not enough to actually stop him. And when his heart stops, his heart goes out to everyone he cares for one last time before he takes that secret to his grave.
Only to meet back up with Sirius and James in another place and have the old ticker filled with affection once again.