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.
The character card with head mode and the one with body mode each have stars. Do those stars all count toward my deck limit?
[Source: WotC] 
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.
If I have a head from one faction on a body from a different faction, which faction is the resulting character.
[Source: WotC]
Both. The character is both an Autobot and a Decepticon.
If I have a character that's both an Autobot and a Decepticon, does my starting team have "only Autobots"?
[Source: WotC]
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).
Can I put a card with head mode or body mode in my sideboard?
[Source: WotC]
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.
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.
Can you clarify timing on when heads activate in regard to finishing a trigger that caused the Body to be
KO'd? Examples: Does
Decepticon Quake damage his own Head when he dies? Octone killing a Body and then his bounty triggers does this kill the Head when it is played?
[Source: WotC]
A head’s deploy ability is a triggered ability. When both players trigger an ability at the same time the active player’s 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 head’s deploy trigger.
This answer is an update on the originally published answer in the Roundup
One character.
Does a card that refers to Bot Mode apply to a Titan Master (Head + Body mode)?
[Source: Aequitas]
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.
You can't bring back a Body mode character from KO because it won't have a head. Effects like
I Still Function! and
Involuntary Promotion will not work at all to bring back a Body. You can, however, bring back a Head from KO because it will be in Bot mode (and only in Bot mode).
Yes.
