Yui's lack of any identifiable HP bar isn't due to a bug in the system.
That's because Yui isn't a player or a monster. She's an advanced program designed to maintain and protect the sanities of players. Her purpose was to act as a mental health counselor to disturbed players.
In other words, she's technically not a human.
Wu Yan rubbed his temples as he started thinking about his Yui's condition.
It's Yui's job to reach out to players and counsel them, helping them sort through their negative emotions. But, went his game turned into a death game, Yui was stripped of the ability to actively reach out to players.
She can only watch as the players she wants to help all fall into hells mired by insanity, anger, hatred, misery, and anguish. She was placed in a situation where she had no power to do her job. Two years after the game started, she finally glitched so hard that she started instinctively seeking out the two players who exhibited the highest value of positive emotions. She broke the Cardinal System's instruction not to approach players to meet the two of them.
In the original work, she was corrupted by all the negative feelings and psychological traumas she saw, meanwhile, Kirito and Asuna were enjoying their married lives so that drew Yui towards them. She was later found by Kirito and Asuna.
Wu Yan isn't Kirito, he had no ill-feelings towards this game. Meanwhile, Asuna also came out victorious with her inner demons. Her perspective changed when she married Wu Yan. The two of them were probably as salient as a shooting star on Yui's psychological monitoring system, radiating positive feelings and thoughts like they were.
Yui appeared on this floor, drawn by the positive vibes they were giving off, except, this time, it's Wu Yan instead of Kirito.
Wu Yan sighed with a slightly worried look. He brushed Yui's hair as he took in her frowning expression.
Yui looked like she's 10 years old but she's actually only as old as this game, which is closer to 2 years. She came into existence as a sub-program by Cardinal System to maintain the mental health of players. She was programmed to be very intelligent, however, in Wu Yan's eyes, she's no less innocent than an infant.
Her fate is similar to Flandre-chan who was locked up, denied contact with other people even when they are just little girls.
Wu Yan rubbed her cheeks, he softly spoke.
"Rest up, when you wake up, things are going to be better..."
Perhaps Yui really heard Wu Yan, she unfurrowed her brows and a faint smile crept onto her face.
Asuna's mood turned better when she saw this positive change. She leaned against Wu Yan's chest as she drew their attention towards Yui.
"She's going to be alright, right?..."
Wu Yan looped his arms around Asuna's waist as he tightened his embrace.
"We don't know who she is but I am sure she's not a monster. Even if we don't fully understand the situation, the fact that she can enter our home means she has her own consciousness. Now, we just have to wait for her to wake up."
Asuna's heart felt a bit better after hearing that. She laid her head against Wu Yan's chest.
"What is a kid like her doing in this game?"
"I don't know..."
Wu Yan closed his eyes.
"Since we found her, we have to make sure she's taken care of..."
Asuna giggled when she heard this.
"As expected, Yan is very warm..."
"Is that so?"
Wu Yan gave her a dirty grin.
"I thought you said I was a brute last night?"
Asuna blushed and she tugged the fat meat on Wu Yan's waist.
"Yeah, thanks for reminding me, you're a barbarian who is definitely not soft with ladies..."
"Oh? I am the bad guy here, huh?"
He hoisted Asuna up and he tossed her on the other spare bed here.
"Let me show you what is nasty!"
"No! There is another person here... Muh..."
"Yan! Yan! Wake up! Yan!"
Wu Yan who was sound asleep got abruptly woken up as he sprung up.
"What?! Is there a fire?!"
Asuna gave the guy a savage knock on the head. That definitely woke Wu Yan up, she yelled at him.
"Dumbo! Buildings can't catch fire in this world, it's the girl, she's awake!"
Wu Yan rubbed his head. He looked at the other bed to see Yui staring back at the two squabbling husband and wife with a perplexed and curious look.
Wu Yan got up and he approached Yui's bed. Asuna also hurriedly walked towards Yui.
Wu Yan helped Yui get up from the bed.
"Are you feeling uncomfortable anywhere?"
Yui blinked her eyes as she tilted her head to think about her answer. She shook her head with a hesitant tone.
Wu Yan knitted his eyebrows, the glitch got to her bad, she's having trouble vocalizing her answer.
Asuna examined Yui like a worried mother. She used a gentle tone with her lest she gets frightened.
"Hey, what's your name?"
Yui repeated her motions from before. She lowered her head to think and then she answered.
"Y-yui... that's my name..."
"Okay, Yui, why did you collapse in the middle of the forest?"
Yui shook her head again. She didn't know anything. Asuna frowned while Wu Yan silently confirmed his suspicions. Her speech isn't the only thing that's impeded. Her memories were affected by the glitch too.
"I'm Wu Yan!"
Wu Yan gave her a radiant smile. He straightened his back as he pointed at Asuna.
"She's Asuna, my wife!"
Yui almost bit her tongue trying to pronounce Asuna's name.
Wu Yan slowed down and he tried getting her to say his name one more time.
"That's Asuna and Wu Yan..."
Yui stopped to reflect and she tried again.
"Awuna... Foo Yan..."
Wu Yan exchanged a laugh with Asuna.
"Alright, whatever floats your boat..."
Wu Yan shrugged.
"Yui, you can call us whatever you like..."
Yui fidgeted with her blanket before she yelled out the title she wanted to say all this time.
Wu Yan had a very exaggerated look on his face right now, his lips started twitching.
"Lo-look, Yui, how about you try calling me Onii-san..."
Yui sported a troubled look.
Wu Yan laughed dryly.
"Okay, papa is fine too..."
Yui beamed at him immediately.
"Yes yes yes...."
Wu Yan tickled her nose.
"Papa is here..."
Wu Yan hoisted Yui up and he turned her towards Asuna who was trying to catch up with the flow.
"Since I'm Papa, you should call her Mama..."
Yui said before she yelled out loud.
Asuna covered her mouth when she saw this little girl call her mama. A switch turned on inside her.
A very beautiful smile floated up her face as she hugged Yui.
"Yes, Mama's here, Yui..."
Wu Yan chuckled when he saw the two girls hugging each other.