To my knowledge the Pulse Knight at the beginning of Episode 2 is a good and easy to upgrade source for the HP+25% passive ability. Others include: Chunerpeton (Yaschas Massif 110AF), Yasksha (Vile Peaks 10AP), Bunkerbeast (Yaschas Massif 100AF), Flandit (Both Sunleth Waterscapes), Luminous Puma (Augusta Tower 200AF), Mewmmao (Archylte Steepe) and Yakshini (Vile Peaks 010AF)
Even though you didn't ask here are some with HP+30% (!!) Bigger investments but better pay off!
Pulse Gladiator (Archylte Steepe), Forked Cat (Oerba 400AF), Necrosis (Yaschas Massif 10 and 1X AF), Lucidon (Oerba 200AF)
As for Siphon Boost I and II it increases the ATB gauge recovery after using a Siphon Ability (Fearsiphon, Faultsiphon, etc) SB I increases it by 50% and SB II increases it 100%
What kind of Butler would I be?