Be somebody’s middle name
Definition 1 of 1
Pro Tip 1/3Personal Traits
Used to emphasize a characteristic that is very strong or prominent in someone. 
Persistence is Tom's middle name; he always finishes what he starts.
Pro Tip 2/3Figurative Sense
It's not a literal middle name but a figurative way to highlight a trait. 
If you ask me, 'adventurous' should be Lara's middle name.
Pro Tip 3/3Positive or Negative
The phrase can be used for both positive and negative qualities. 
Messy is definitely his middle name, look at his room!