It should hurt to move, but it doesn't. It's amazing what adrenaline will do.
He is half way up before he has even processed the first impression, the round, vertigo-inducing tower rising above his face as he opened his eyes. But he's on his way now, and everything in him is red and orange, a dull roar, hushed but louder, louder than the din of battle outside.

Would you ask him why he is in such a hurry? You could, I suppose, if the howl of the draught through the hollow structure would carry your voice as far, but you might not get a response, and certainly not a well-reflected one.
His name? His name is Cassian. You may, or may not, have heard it before, but I assure you, you know him. You've heard about him. Public Enemy no. 1, they will say, and that's who they mean.
And they would be right, though this is a soft-spoken man. A quiet, observing man. Wariness has travelled with him as far back as he recalls, but it has left his body now, and been replaced by urgency. What is so important up there?
Back inside we go, and into the red, into his pounding heart and the hush of the blood. There is a face there. Can you see it?
He's bleeding, and outside is a battlefield, and it's a long, long way down. But he's been a walking dead as far back as he recalls. Fear has been such a constant companion for so many years, that it has become familiar, a tight, invisible belt around his torso. He'd feel naked without it. His stare is focused, his face intent, calm like marble, it belies the roar within. This is the stare the Alliance has sent, many times, to watch a target from far, far away, or above. These are the hands who held the rifle steady. It is long since they forgot how to shake.
Last time he climbed, high and madly, like now, he left someone behind below. Someone he killed. It wasn't someone who had done anything, yet, and they might never have, but the chance was not his to take, on behalf of so many souls. He soothed his comrade, a final kindness, and he pulled the trigger, and he bled inside. He had a message, information, a purpose to protect. And he must be that purpose, and one day someone else might make the same decision about him.

This climb is different. There is no torch to pass on and protect… the torch is long since gone, ahead and above. And yet, he climbs. It is a climb towards, not from. A climb to reach. And that makes all the difference.

A crooked smile and a small, low 'hah' escapes him, as he stares at the chute barring the way further up. It is a deathtrap, and he sways underneath it, taking in the rythm of its opening and closing, a greedy scissor waiting to cut the thread of his life.
It is a young thread, strong and straight and dark red with blood and purpose, tasty indeed.
But he'll pass through that maw. Because Jyn must have passed through it, and she is up there.
Jyn, that is her name. Didn't you know? Oh, rebellions are built on hope, sure. But tell me, what do you think drive them? Drive them forward?
Affection? Love? The Force itself?
Many names.
Whatever you call it, that is what is fueling this man now. He is brimming with it. It flows out of him. Kindness, gentleness, is a quality so basic to who he is, how he was made. But until so very recently, there has been nowhere, no how for it to unfold. It sat there, sheltered and protected, like the contents of a walnut, folded up inside a smooth, hard shell. It somehow did not go rancid, somehow did not dry up, and this, in itself, is an act of rebellion.
And now is the time for the seed to sprout.
Climb!

He had one friend. This was his friend's last, desperate advice. Not his advice; his charge.

Climb. Climb!
His friend was an Imperial droid. Cassian reprogrammed him, and made a friend. Literally. Now, K2-SO lies below, fallen in battle while sheltering his comrades. Like a cracked shell, he lies down there, and there is no way back to him.

The man sways, breathes, readies himself for that quick, hard exertion which he knows will probably hurt. Though if all goes well, not as much as being cut asunder.
Up it shoots, the sapling, defying the scissors and the cold, howling vertigo that should have been its tomb. And at least for now, it is not cut.
He moves quicker now, his hands are offshots, running upwards and and taking hold, he knows the direction of the daylight. He does feel naked now, and realises it is because it is gone, that tight belt of fear around his chest is gone, and it is like he is expanding, and green, and maybe he will be mown down but it doesn't matter. It doesn't matter, because he has taken root in a purpose that belongs only to him, a direction which is his alone.
When he emerges into the fresh air, it feels almost abrupt. He sees the woman, and the white-clad threat. The blaster, the calm, steady aim who had forgotten how to shake, does the job for him. The Imperial officer collapses in a heap, down but not dead. For once, Cassian is ready to abandon caution and forget all about it; there are people more importance in the vicinity.
She is in a fury - this officer is the man who took away her life and her family, and she howls, an animal in pain, advancing to maul, to tear limb from limb. But Cassian remembers that rage. He also knows how you feel afterwards, if you give into it.

A final offshot reaches, wraps around her, pulls her in.

Jyn. Don't. (You have family. You have me).

He breathes into her hair.

He's not worth it, Jyn. (He remembers the rage alright, but it is a memory, now. He has become all growth, and life, and lucid).

And she gives up, or gives in, relaxing warm and heavy against him, which is only too well, because this is the moment the pain, finally, returns. He closes his eyes, and leans back in, accepting her offer of support. He knows, now, that they will both die, but it doesn't matter. She is dirty, and warm, and soft, and so is her voice. Hoarse and smoky and raw.

Rebellions are built on Hope, but this is what drives them.

This is how trees begin.