Here’s their playlist of their many item trivia videos, which include the (so far) six videos on Assist Trophies (16 videos, total length, about 6 hours):

Part 1 – Part 2 – Part 3 – Part 4 – Part 5 – Part 6

Just some of interesting facts revealed about Assist Trophies:

• Some Assist Trophy characters have their own HP, and if players deplete it with attacks they can even be KOed with a strong enough attack. This earns the defeating player a point, although it doesn’t cost the player who picked up the trophy a point.
• Usually the owning player can’t attack their own trophys, but if the trophy has started glowing, a state they get into when very low on health, the trophy will actually become vulnerable to attacks even by the player who collected it. This is so they can earn the point before another player earns it.
• The trophy with the highest health is Mother Brain, with a base health of 120 HP.
• “Base health,” what’s that? Assist Trophys that have health get more health the more players are in the game! This goes up to twice their usual health if there’s eight players.
• A good number of trophies have health, and can be attacked, than you’d might think. For example, in the trophy of Color TV Game 15, an early Pong-clone Nintendo made in 1977, the paddles are vulnerable to attack. Each has only 15 HP, but the trophy is defeated if either paddle is defeated.
• The Starman trophy, if it’s defeated, displays the SMAAAAASH!! message from Earthbound!
• Each Assist Trophy has a list of stages is can’t appear on. Very few trophies can appear on absolutely any stage. One of them is Lyn from Fire Emblem.
• There are even trophies that get blocked by Omega or Battlefield versions of certain stages. Usually these are trophies that reside in the background in some way, like Midna or the Moon from Majora’s Mask.
• Each Assist Trophy also has a list of items, and other Assist Trophies, that it disables, that can’t generate while it’s active, and alternatively, if that item or trophy currently exists in the world, that prevent it from appearing.
• Referring to that last point, “existing” can mean many things. Like, an item held in Isabelle’s pocket still exists in the world, and so will block all the items that can’t generate while it’s instantiated almost indefinitely.

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