Two things: First, Chara can't be seen or heard (with exceptions) but can be felt. People can touch her, and she can touch them.
Second, If you have any questions, feel free to ask. I can't answer everything due to not wanting to spoil anything, but I'll happily answer anything I can. For example, If you want to know why Chara refers to themselves as They/Them, but everyone else uses She/Her, just ask, and I'll happily answer when I get the chance.
'Another reset?' thought Chara as they quickly looked Riley over. They found the new death mark in the center of the three-year-old's chest. 'A target?'
"So, mind explaining what happened?" demanded Flowey as he popped up out of the ground. Chara was too busy thinking about why the death mark was a target.
'Is it because she wasn't on guard? Riley is basically a walking target for the other monsters,' thought Chara. They were brought back to reality when a vine whipped them on the head. "When did you get here, Flowey?"
"What happened?" repeated Flowey. He then turned to Riley, who was happily walking around. "Also, how are you so energetic after dying twice?"
"Dying?" asked Riley.
"You're too innocent," complained Flowey. "It ruins the fun."
"I...left Riley with Toriel," said Chara. "Riley, could you tell me?"
"Hmm," hummed Riley as she thought about how to tell her mom. "Bed."
"So she put you into a bed," said Flowey.
"Pie," continued the child.
"She went to do something with a pie," figured out Chara.
"Chara Mom," said Riley as she smiled at the ghost.
"You snuck out to find me," said Chara. 'If I didn't leave you, you wouldn't have died so soon.'
'Ms. Naive here probably accidentally triggered the black stuff by calling Chara mom,' thought Flowey.
"Rib...ribbit!" shouted Riley as she leaped forward to hug Chara.
"A froggit attacked you," finished Flowey as he thought, 'so Riley's unconditional love for Chara can also quell that black stuff. How disgustingly cheesy.'
"Hello," said a familiar voice. Flowey quickly disappeared underground as Toriel entered the room. "Oh my, are you okay?" Toriel promptly ran over and began checking Riley for injury. 'what are these marks?'
"Hehe," giggled Riley as she tried to get away from the motherly monster's fur that tickled her.
"Child, please stay still," said Toriel as she tried to keep the squirming child still.
Chara smiled at the scene before gripping their head in pain.
"Mom, stop, you're tickling me," said Chara as they squirmed in Toriel's grasp.
"I need to make sure you're okay after you tripped," said Toriel as she kept Chara from getting away.
'what was that?' wondered Chara. 'was that a memory?'
"Other than those marks, you seem fine," said Toriel. "Why don't I bring you back to my home, and we can eat some pie?"
'I hope she doesn't mean snail pie,' thought Chara as they followed the two.
"I could even make you some clothes to wear," said Toriel with a smile.
"Clothes?" asked Riley as she tilted her head.
"Clothes are what you wear to cover up, like my robe," said Toriel.
"No clothes," pouted Riley.
"Why don't you want clothes?" asked Toriel.
"Mom no clothes, Riley no clothes," explained Riley. Chara looked down at their nude form and sighed.
'I don't wear clothes sometimes because it doesn't matter for me,' thought Chara. 'I can't get sick, and the only people who can see me wouldn't really care about my state of dress.' Chara watched as Toriel explained why Riley should wear clothes. They formed a simple black dress that reached their knees using the black stuff. "Riley, look."
"Ahh," gasped Riley in amazement. "Mom robe."
"Your mom is wearing a robe?" asked Toriel. Riley nodded.
"This is a dress, Riley," explained Chara.
"No robe, dress," said Riley with a nod.
"Does your mom look beautiful in her dress?" asked Toriel as she thought, 'is someone else actually here, or is this child just imagining things?'
"Beeutefull Mom," said Riley with a smile.
Chara clutched their head in pain as Toriel taught Riley how to say beautiful.
"What would you like, my child?" asked Toriel.
"I don't need anything," said Chara. "I already have you guys. That's all I need."
"Chara, it is fine to ask for something," said Asgore after sharing a look with Toriel. "I can't promise we will get it, but I promise that we won't get upset."
Chara remained silent for a couple of minutes before speaking, "Can…can I have a dress?" before flinching away in fear.
"A dress?" asked Asgore. "Is that all?"
"Huh?" asked Chara in confusion. 'They aren't angry or disgusted. They gasped in surprise as Toriel picked them up.
"I can see it now," said Toriel as she held Chara in her arms. "A beautiful dress for my beautiful daughter."
'Why aren't they upset?' wondered Chara.
"Why don't we make a dress for Asriel as well?" laughed Asgore.
"No way, boys don't wear dresses," said Asriel. Chara began to shake in fear as memories of their surface began surfacing. "I'll leave the dress wearing to my sister."
"Now now, there's nothing wrong with a boy wearing a dress," said Asgore. "It certainly isn't common, but it is fine for it to happen."
"WHY?!" shouted Chara as they dropped out of Toriel's embrace. They sobbed as they yelled, "Why are you being so nice? Why aren't you disgusted? Boys aren't supposed to wear dresses. It's unnatural. I'm a freak, so why do you love me?" Chara curled up into a ball and continued to cry.
"Because you're our child," said Toriel as she wrapped her arms around the sobbing child. "Nothing will change that."
"Chara is my best friend and sibling," said Asriel as he joined the hug. "I don't care whether you're a boy or a girl."
"If someone thinks your gender or how you dress makes you unworthy of love," said Asgore as he wrapped his arms around the three, "then they are unworthy of being a memory of yours."
"Mom?" asked Riley as she tugged on Chara's hand. Chara gasped as they were brought out of the memory.
"I'm okay, Riley," said Chara as they wiped the tears from their eye sockets. Ruffling Riley's hair, they said, "Let's go and get you some clothes."
"Riley dress," said Riley as she held onto Chara's hand.
"I'll make you a dress when we get home," said Toriel as she stared at Riley's hand. 'Something definitely touched this child's hair, and now she is holding something. Is her mom actually here?'