Jenna hadn't told her mother she was seeing anyone because she didn't want her to worry…or ask questions she wasn't allowed to give answers to. She couldn't refer to anyone by name.
Killian was "my case agent." Josh and Del were "my coworkers" and if she had to refer to them specifically it was "my guy coworker" and "my girl coworker." She could call Bryce "my boyfriend" but she was worried about the other inevitable questions.
She couldn't say what his job was or where he was from. She could talk about things they did together but what if she slipped up? She had already nearly blown it by talking about her twin when she was half asleep; if she was gushing to her mother about how great he was, she might accidentally use his name.
And what if things didn't work out? They probably wouldn't. She would be going home in February and he would stay here. There wasn't really a point in talking about him, was there?