When things started looking dire in Heavensward, right before the field trip idea was brought up by Alphinaud, Theodore actually asked Mordred to take the kid and Tataru and leave.
Theodore himself couldn't leave. Ishgard was his home. For all it had taken from him, for all the bad memories he'd had here, it was his home. He already fled it once, and could not do it again now that the Horde's attack was imminent.
"But it is not your home," he told Mordred, "and you need not stay. Your future lies elsewhere, Mordred. You must carry on, and preserve that which Minfilia and our friends gambled their lives to preserve."
It was a low blow and Theodore knew it. He wasn't there in the tunnel with Mordred. He was in Coerthas -- had been for months, fearful of another attack after they pushed Vishap back. He just knew whatever happened there hurt Mordred immensely, almost as much as being blamed for the sultana's assassination hurt him.
But Mordred hadn't gotten mad. Actually he did kind of swell up with rage for a second...but as Theodore braced himself for what he was sure was the end of their friendship, Mordred deflated again.
"I'd rather stay and face the Horde with you than try my luck alone out there," he told him. "There's not very many I can trust, Theodore. You're the last one."
"Don't say that. You make friends wherever you go. You will be fine," Theodore replied sharply, but doubt was already growing.
Mordred looked at him with enormous eyes. The kind of 'fuck you're so stupid but I'm gonna act cute about it' look. "Aye, everyone will love to reconnect with the bastard with two million gils bounty on his head. And then they'll get me and who will bail me out, huh? Alphi? Alone? Yugiri's tricks won't work twice now that we already sprung Raubahn."
"You cannot stay here and face the Horde. The risk is greater than you can imagine!" Theodore snapped.
"Can you?" Mordred retorted.
"I cannot leave--"
"Then the only choice we have is to win. It is the only choice." Mordred shoved the heels of his palms into his eye sockets. There was a moment of tremulous silence before he removed them, took a deep breath, and grabbed Theodore's wrist.
He said, "Don't ask me to leave you behind. The future Minfilia and the others entrust me with has you in it, too."
He said, "I need you to stay -- stay with me. Please."
It was impossible to refuse him.
So Theodore did not.