This is a little back story so the next part of the book will make sense to le readers.

Liam was Logan's older brother by about 2 years. When Logan was 8 his disappeared, when his mother dragged him down to the basement.

Liam said he'd always protect Logan, no matter what.

When Liam didn't come back -around half a year - because Logan was such a lonely child he started to imagine what Liam would say to him. [That's also the underlined Itallics in Logan's mind.]

Liam was a good brother but kind of vicious to anyone who got near his little brother as his mother used to attack them - he was suspicious of everyone and didn't know how to act socially. (Neither does Logan) So Liam would volunteer/sacrifice himself for Logan to be safe.

But over the years, in Logan's mind, Liam is still there and kicking. He brings out the more vicious/pyschotic side of Logan 'cos when anyone went near Logan that looked suspiwto Liam - he would beat them up and tell them to stay away. Logan got the 'sense'/ natural instinct from watching Liam as he grew up.