environment

• vim: 7.4
• OS: Debian Jessie
• terminal: XTerm
• $TERM variable: screen-256color-italic
• color configuration (:set t_Co?): 256
• font: DejaVu Sans Mono for Powerline Nerd Font Complete Mono.ttf

Hello,
A square character is appeared near line indicator, and I found g:airline_symbols.maxlinenr keeps that char and I am able to correct it from .vimrc, but I wonder why its default value is look like square.
That character is '☰'