It's decided to be Gastly.
The fear they felt stemmed from the unknown. Once Anko, Kurenai, and Naruto realized that Gastly was merely playing a prank, the fear in their hearts quickly dissipated.
In fact, the fear that Anko and Kurenai had wasn't that deep. It was more surprise than actual fear.
"This is incredible, Gastly! You just vanished, and then reappeared out of nowhere. How did you do it?" Naruto asked excitedly, his face nearly pressed against Gastly's.
"Right? And that punch I threw went straight through it! It's like space-time ninjutsu," Anko added, thoughtfully touching her chin.
Even though she had accepted Gastly's existence, Anko was still filled with curiosity. The idea that there were 'ghosts' in this world was both amazing and absurd.
"Gastly is a Gas Pokémon," Fuji explained, chuckling. "Its body is made of gas, so it can pass through anything. Anko, of course your punch went right through it."
Fuji's explanation seemed so matter-of-fact that Anko gave him a playful glare. She couldn't believe she had been so startled by something like this—how embarrassing.
"You're such a naughty little ghost," Kurenai said, reaching out to touch Gastly's cheek gently. The sensation was strange—like touching air.
"Gaaaastly! Gaaaastly!" Gastly laughed, remembering how it had scared them all earlier.
"Don't get too cocky," Anko said with a smirk. "You just caught us off guard. Next time, we won't be so easy to scare."
Gastly's expression shifted into a smug grin, as if to say, "Really? Who was just shaking in fear a moment ago?"
Anko's face reddened slightly as she prepared to retort, but Fuji stepped in.
"Alright, alright. Gastly, stop teasing Anko. And Anko, no need to get competitive with a Pokémon," Fuji said, separating the two before things escalated.
Gastly, ever the prankster, obediently quieted down at Fuji's request, hovering innocently beside him.
"Wow, it really listens to you, Fuji," Kurenai remarked, impressed. She had assumed that such a mischievous Pokémon wouldn't be as obedient as Anko's Snivy.
"Of course! I'm the manager of Pokémon House, after all. Gastly knows to listen to me," Fuji replied with a grin. "So, Kurenai, what do you think? Do you like Gastly?"
"I do like Gastly, but... will it listen to me as well? I don't want to be pranked every time I come home," Kurenai said, smiling softly as she looked at Fuji.
"Gastly," Fuji said, turning to the Pokémon. "Kurenai is going to be your trainer from now on, so you need to get along with her."
"Gaaaastly!" The ghost Pokémon floated over to Kurenai, nodding its head to show its obedience.
"Gastly is adorable. I like it," Kurenai said, gently patting the Pokémon. "But is there anything specific I should be aware of when raising it?"
"Gastly is definitely a unique Pokémon," Fuji began. "Its body is made of gas, which is why it's classified as a Ghost-type. It can sneak into any place it wants because of its gaseous form. But when exposed to strong winds, its body can be scattered. Luckily, it can usually regenerate afterward."
"Gastly can also surround its prey with toxic gases. It's said to be able to bring down an Indian elephant in just two seconds with its poison."
Fuji continued to explain, recalling various details from his memory. In some cases, Gastly even gained the ability to speak like a human, communicate through telepathy, and transform its form. These abilities had been shown in different Pokémon stories involving ghosts and supernatural events.
"Gastly is capable of many unique abilities. It can create illusions, hypnotize unsuspecting prey, or even use its poison to drain energy from others."
"Most often, though, Gastly likes to live in old, abandoned buildings or cemeteries. But it's perfectly fine to raise one at home, as long as you give it the proper care."
Fuji wrapped up his detailed explanation. "That's about it. You can also use your Pokédex to scan Gastly and find out more information."
Only then did Fuji realize that both Anko and Naruto had their own Pokédexes, and they could've used them from the start.