When in the overworld, a small portrait of both Ren and Stella appear with their respective HP bars and Esper/Overheat gauges. The one you are controlling currently has their bars sitting on the bottom right of the screen, while the other character's is up on top. According to the situations in battle, these portraits will change. Here's a couple of these "Emotions".
*Note that due to story circumstances, Stella won't be able to immediately take advantage of or be hindered by as many emotions as Ren can at the start. Once certain points are reached, she'll slowly learn them by watching Ren, except for Full Synchro, which both characters start out with.
Normal: Default mood with no outstanding effects present. When out of battle, characters are relatively calm. When in active battle, Ren puts on either a determined glare or a confident grin depending on the strength of the enemy relative to himself, while Stella emits blue flames from her left eye regardless of enemy strength. When hit with an attack, the character portrait reacts according to the strength of the hit, from glancing hits to full on body crashes.
Full Synchro: When the Synch bar is full, the player can activate Full Synchronization (shortened to just Full Synchro), allowing Ren and Stella to coordinate their attacks as one to make them more powerful. In this state, Ren and Stella have a flaming aura respective to themselves and a pink line linking both characters. Their portraits have white black and blue auras respectively, their eyes pulsing between Ren's gray and Stella's sky blue, and both bars are merged together to form one big Synch bar that steadily decreases over time and decreasing faster when getting hit. When Synch runs out, both return to Normal.
Anxious: When Ren (and eventually Stella) take multiple hits from foes without successfully retaliating, they become worried. Full Synchro and Synergizing are disabled until the player manages to score a few good hits of his own back at enemies. Ren looks at the player with gritted teeth that says "What're you doing dude?" while Stella lowers her eyes a little and looks at the player worriedly.
Dark Esper Soul: Ren Only. As a side effect of Synergizing and/or Over-Synergizing when he's in Tired state, it is possible Ren's focus will slip and he'll end up in Dark Synergy. The player loses control of Ren in random intervals while he attacks both enemies and occasionally Stella! She's able to calm him down quicker if she's near him, but it's also risky since his attacks HURT. After Stella manages to get close to him for a while or until the form time runs out he'll revert back to Exhausted state. His overall appearance takes on a black shadowy form with solid glowing red eyes. His portrait reflects this, with Stella's looking up or down at his with a determined face*
Angry: If Ren or Stella take a huge amount of damage in one hit or broke free from a status effect without any effort in reducing its duration, they enter Angry state. In this state, they will ignore flinching and blowback attacks and the next skill triggered has triple the damage or effect. Unlike Full Synchro, Angry only dissipates with time. It can be triggered multiple times in a battle if facing a really strong enemy, but of course, you're also losing massive chunks of health, so watch your temper! Ren's Angry portrait has rising red flames with his eyes glowing bright red with a frustrated facial expression, while Stella has rising blue flames, with her looking deeply intimidating with a furious blue and white vapor trail coming from her eye.
Low HP: Just as it says, the character on their last legs. The HP bars flash red and yellow with different alarms sounding for the primary or secondary character. Ren's portrait has his face flushed of color with him looking downward, cuts bruises blood marks and gashes all over his face while Stella's has her eye flame becoming weaker and her face very strained with cuts and blood marks all over her.
Pinch: In a Pinch happens when you perform poorly in battles, such as taking multiple giant hits and falling to Low HP. The portraits are the same for Low HP, except a noticeable red outline appears on it to show you who's suffering.
Dead/KO: Same as low HP, except the character portrait is shadowed with a big red X over top of it.
Tired: A state that appears when Ren and/or Stella reverts from Synergizing. Synergization can only be used when the appropriate energy is stored up, and even then in most cases it can only reasonably used once per battle before becoming Tired. In this state, Full Synchro cannot be achieved but Synergization is still possible for Ren provided he still has Esper in reserve. The problem is that each subsequent Synergization has higher chances of Ren triggering Dark Esper. The state wears off after a while, so it's safer to wait until then before attempting Synergization again. Ren's portrait depicts him panting while sweat drips down one side of his face, Stella's has her eyes lowered breathing heavily with her eye flame barely sparking
Synergy: The powered up state achieved from using Esper Stones. In this form Ren (and Stella if Ren decided to temporarily infuse her with it later on) become significantly stronger, regening HP and a host of other effects depending on how many stones you restored, with the highest possible strength achieve when all 7 are restored. Ren's portrait shows him with his arms crossed and glaring straight ahead with shimmering sliver up-turned hair and glowing red pupils. When Synergized whilist Tired, he has a more strained look with sweat running down his face and a red number detailing how many times he's exerted himself this way. Stella's has her black hair partially turn white and a little more upturned, her eyes staying the same with her flame now a shimmering and brilliant blue-white.
Exhausted: Ren only. Coming back from Dark Esper has put so much of a strain on his body that he can't even move properly for a short time, with his HP rapidly falling to 1, all his attacks doing pitiful damage. Move Stella to the frontlines and use her to deal with any foes, because Ren will be as durable as wet paper for a long while. His portrait had him doubled over with sweat soaked all over his face and in absolute pain.
Sadness: Stella only. After a certain event, Stella will have this state for a while. It's almost the same as Ren's Exhausted, but doesn't last nearly as long. She has her head hung low, looking defeated.
Cold: Ren only. He's not a fan of this weather! It's purely for comedic purposes while in Stage: Moscow. He has his arms wrapped around himself, small icicles forming on his hair spike tips with a face that just screams "COLD!!!"
Underwater: Ren only. Using this will help you figure out how much air Ren has left before he needs a fresh breath. The portrait is normal until 75% when a slight blush appears on his face with his cheeks puffing slightly. At 50% his face turns a bit blue with his eyes narrowing a bit. At 25% his face is completely blue, his cheeks are puffed up, and he has a look of complete terror on his face.
Love(?): After a certain event and Ren and Stella have high Bonds, this state may appear if they are near each other. A slight HP regen effect is applied when present, and negative status effects dissipate quickly. Both character's look at each other while a soft pink glow emanates from the Sync bar. Ren has a hand behind his head with a bashful smile, and Stella has a soft smile with a blush with her eye flame dancing happily.