From the Super Mario Wiki
“Lady Bow, your beauty is like the song of a nightingale in the evening.”
Bootler is a Boo butler who works for Lady Bow in Boo's Mansion. He also acts as her personal advisor. Bootler is very over-protective of Lady Bow, and initially opposes her joining Mario on his quest to defeat Tubba Blubba (and even then wishes to go along with her, though she does not acquiesce). This protection to his mistress makes him similar to Toadsworth.
"Bootler" is word play off of "Boo" and "butler". Goombario actually states that "the correct term is 'butler'" upon tattling Bootler, indicating that the portmanteau in his name may have simply been caused by the unique Boo speech patterns seen exclusively in Paper Mario (which do indeed involve replacing words and parts of words with "Boo").
Bootler seems to have a certain connection with Lady Bow's ancestors. He constantly reminds her that the ancestors would be displeased if she was harmed in any way. According to a seemingly knowledgeable Toad found near Toad Town's port, Bootler was once in love with Lady Bow's grandmother, who was apparently Boo's Mansion's mistress during Bow's early childhood. This Toad also reveals that, in his youth, Bootler traveled the globe with Goompa and Koopa Koot.
Bootler has legendary status as a master frightener. He even scares Fice T. and Mario (and possibly the player, too) at the entrance of Forever Forest. When he frightens people, he grows many times his normal size and screams completely suddenly and unexpectedly. Bootler seems to enjoy scaring Fice T., as implied in the letter he sent to him.
 Names in Other Languages