Does Sett Kingpin get his +1 buff when attacking a battlefield, or only when defending one (since the text says 'my battlefield')?

Ruling: Sett Kingpin's buff applies whenever he is at a battlefield, whether attacking or defending.

Nuances:

• The 'my battlefield' wording is used to prevent him from getting the bonus when he's in base, not to restrict it to only defending battlefields.