If I cast a Lightning Helix targeting my opponent's Gideon, Ally of Zendikar on his upkeep with it having 3 Loyalty, can he in response use Gideon's plus 1 ability to put it out of Lightning Helix's range?
2 Answers
No, a player can't activate loyalty abilities during his upkeep nor in response to spells or abilities. Loyalty abilities can be activated at "sorcery speed", only during a player's main phase. The specific rule from the Comprehensive rules is:
606.3. A player may activate a loyalty ability of a permanent they control any time they have priority and the stack is empty during a main phase of their turn, but only if no player has previously activated a loyalty ability of that permanent that turn.
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Teferi, Temporal Archmage is the only exception to that rule, but it is stated quite explicitly. Commented Jul 2, 2018 at 20:17
You can [Only] if, you have either Teferi, Temporal Archmage's <-10>Emblem or Teferi's Talent Enchanted Planeswalker's <-12>Emblem.
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For what it's worth, we've generally found that answers to MtG questions saying "unless you use (insert specific card here)" are generally not that well-received, unless of course that is specifically what the question is asking for - MtG is such a broad game that there's a card which gives an exception to every rule. Commented Jan 14 at 17:10