"Hard realization to avoid."

"Some manage it anyways."

"I'd like to think I'm a little smarter than those people."

"You have been so far."

"Thanks, sensei."

"Only the truth."

"What can I do to protect myself?"

"You can use barriers, especially if you know you'll be in one location long enough to be worth it."

"How do those work?"

"They imbue your spiritual power into an object or area, keeping those you don't permit - and who are weaker or less clever than you - from crossing the boundary you set. Most humans use paper with incantations written on it."

"That could be useful for the house."

Snicker.

"What?"

"I already laid some wards, as your bodyguard."

"Are they any good?"

"Brat!"

"I think it's a reasonable question to ask."

"Pah. And for your information - I'm one of the more powerful, skilled youkai you'll ever meet. But - this is not my territory, so my ability to ward it is... Looser, than yours would be."

"So they could be better."

"They could be more thorough," she corrects. "Right now, I'm having to leave gaps for all those youkai you're constantly inviting - and I can't use the strongest barriers, because you're always letting random youkai in, and I'm assuming you don't want to fry anyone seeking their name back. The barriers will forcibly reject any youkai with ill will - but that's easier to trick than a barrier that will forcibly reject all youkai. And any youkai invited in by a true inhabitant of the home will be able to bypass the barriers entirely - I can't make it just respond to you, but that means any youkai clever or powerful enough could trick the Fujiwaras to get in."

"Can we make things stronger just temporarily? Until we deal with this youkai?"

"Yes. And you can place a second, tighter barrier under mine."

"Then let's do that."

She'll run over some of the theory with Tsubaki first. 

It's a little esoteric, but not too complicated...