Jenny had been raised to be careful what she wished for. She didn't want him to have a 'dark side' per se, she just hoped there was something beyond the tweed and ceremony, something she could really get excited about. Sure, he was intelligent and gorgeous and kind, but even with his passion for books there was a certain edge missing, a fire of sorts. (Though, ironically, she enjoyed mentioning fire around book-lovers to see the fleeting panic in their eyes.) She'd found his tattoo when he (finally) invited her over, and she could see the faint traces of grown-in ear holes when she kissed his neck; she knew there were rebellious youth stories buried somewhere inside him. Maybe things she'd like to see reemerge, but maybe not.
He'd kept her at a distance since the Master rose. However sincerely interested she was in his drama with the Council, it never felt like he was telling her the whole story. He'd been promoted to Watcher after completing his eight years of training, but for the last ten years he'd been strung along with no actual Watching duties. After his brief apprenticeship with the Watcher of the London slayer (after her death, he'd noted), he'd been sent to America to work with Merrick in Sunnydale. But when the new slayer was rerouted to the more active Cleveland hellmouth, Giles was to monitor the secondary in a purely informant capacity. He had indeed notified them of the Harvest, but was unable to stop the Vessel due to sheer lack of force. Even reports of the Anointed and the Master's prophecies weren't enough to transfer the Slayer to his watch. And when the news of the Master's rising had reached the Council by other means, Giles had been fired for apocalyptic negligence without a moment's hesitation.
He had taken in an apprentice of sorts on Jenny's recommendation, a girl named Willow whom she herself had recruited for some special projects after school. After easing her into the whole techno pagan deal, she was pleasantly surprised to discover Willow had a genuine interest in fighting demonic forces with them. They had first worked as a team to defeat Moloch, but afterward Willow seemed to spend more and more time in the library. Giles had sporatically mentioned a boy named Xander, Willow's best friend, but as he avoided the library, Jenny had never met him. Apparently Xander had lost a friend to the Master earlier in the year, and then he himself was sired after the Master rose. Jenny didn't push Willow to talk about it, and even Giles wouldn't bring it up lest his emotions get the best of him. His growing paternal love for Willow wasn't fully realized until she, too, was turned.
Jenny was fairly certain that's when the drinking started.
She'd never thought he could drop so far over the edge. It was as if she'd never seen pure despair until the morning she found him slumped in his office over an empty bottle of brandy. Something inside him, some undefined breaking point had been crossed, and he was no longer a man of reason. She had tried (though, admittingly, there was always more that could be done), crossed to meet him halfway and provided the tools to dig himself out. Unconditional support, conversation, a safe location, even love. But as the months passed, she realized more and more that the man she knew was lost. Drowned in wine and self-loathing. It was almost as if he'd been turned himself, she mused, the darkness having taken him over completely. And for all her efforts, the year of standing by his side, there was nothing left for her to hold. She didn't bother to wake him up when she came to say goodbye; he deserved a good dream as much as anyone.