Glint-Sleeve Siphoner

The first triggered ability triggers both when Glint-Sleeve Siphoner enters the battlefield and whenever it attacks. You don't have to choose only one.

Some triggered abilities that state that you "may pay" a certain amount of {E} describe an effect that happens "If you do." In that case, no player may take actions to try to stop the ability's effect after you make your choice. If the payment is followed by the phrase "When you do," then you'll choose any targets for that reflexive triggered ability and put it on the stack before players can take actions.

If an effect says you get one or more {E}, you get that many energy counters. To pay one or more {E}, you lose that many energy counters. You can't pay more energy counters than you have. Any effects that interact with counters a player gets, has, or loses can interact with energy counters.