Titan Masters Attack introduces two new character modes that work together: head mode and body mode.
Cards with a head mode are placed above a character that has a body mode, combining them into a single character. The resulting character is a single character made up of both cards. The body mode card will tell you the character's stats and abilities, and the head mode card will give the character a bonus. That bonus may be an added ability or an increase to attack, defense, or health. Like other characters, that character starts the game in alt mode. If the character is KO'd, even if it's your last character card, the card with head mode is returned to the battlefield in bot mode. That bot mode character can't flip.
Yes. Each card's stars count toward the 25-star limit. During the game, the character has all the stars from its two cards.
The body mode card can flip back and forth, from alt mode to body mode. The head mode card doesn't flip, so its bonus always applies, no matter which mode the character is in.
Yes. They're completely mix-and-match. You could even put an Autobot head onto a Decepticon body, or vice versa.
Both. The character is both an Autobot and a Decepticon.
No. In order to qualify as "only Autobots," your starting deck can't have any characters that are another faction (even if they're also Autobots).
You can but note that each card is a separate character card. This means you could have a character card with a head mode or one with a body mode in your sideboard, but not both.
Card requirements are inclusive, not exclusive. This character fulfils both the Autobot and Leader requirements on Matrix of Leadership, so you may play the card on the character and the 'While on a Leader' ability will function.
Cards try to do as much as they can. You can play Unconventional Flying Object even if you don't have a Spaceship and it will only accomplish putting the Action into your scrap. There are a handful of Upgrades like Crushing Treads that are restricted to play on say "Put only on a Tank." - you can't play that card unless you have the specific kind of character to put it on. After you successfully put it on the right kind of character, the game won't check anymore if the upgrade is valid, so the character can flip to Bot mode, stop being a Tank, and the Upgrade will stay on.
An important rule to remember: The active player chooses in which order to resolve simultaneously triggered abilities.
A head's deploy ability is a triggered ability. When both players trigger an ability at the same time the active players abilities will complete before the defending player. A player may order their own simultaneous triggers how they wish. The player controlling Quake may order Quake's damage trigger and Quake's head's deploy trigger such that Quake's damage ability resolves first, and Quake's head second. If Octone's controller is the active player, they must use Octone's bounty trigger before the defending player resolves their heads deploy trigger. This answer is an update on the originally published answer in the Roundup
An important rule to remember: The game checks for a player controlling zero characters at all times. However, Head character's deploy ability includes the following: 'When that character is KO'ed, even if it was your last character, deploy this battlefield in bot mode.' This is why you don't lose if your last character, a Body with a head, gets KOed.
2023-11-03 Update: This has been further clarified in this ruling
No. A character in Body mode is not also in Bot mode.
This means that cards like Escape Route do not flip a Body mode character to Alt mode. For an example of a card that works with both Body mode and Bot mode characters, look at how Mission Briefing explicitly says 'in bot mode or body mode'.
Note that cards like Start Your Engines and Hunker Down do not flip the characters from Body mode to Alt mode, however if the characters have Car and Tank traits respectively, they still get the secondary effects of those cards.
At the start of the game head and body characters are presented as two separate characters. This means "Stuck on you" and "I've got your back" won't be active if you have a head + body character and another character on your starting team.
Heads and Bodies are separate characters in your starting team. You attach the head after your starting team has been presented, and before "Before the game begins" triggers.