That voice sounded familiar, and what exactly did he mean by, 'my replacement'? Then you put the pieces together.
"Jason?"
*****
Your smile faded when you saw the elevator open and the Batcave was revealed beneath Wayne Manor.
"<y/n>?" Jason's voice asked quietly when you'd been silent and frowning for a while.
"You're Robin?" You asked, and when he nodded you looked away, "That explains a lot"
"Aren't you happy for me?"
"Of course I am!" You replied, looking directly at him, knowing that he could see the tears in your eyes "I'm just... Worried about you. Even more than when I didn't know what you were always doing."
"If you're so worried, why don't you join us?" He asked hopefully as he led you further into the cave.
"Join you? Like, fighting crime every night?" You asked. "I- No. You've obviously been doing this for years... I'd just get in the way"
"Not necessarily, you do Gymnastics. You already have a head-start!"
"Look Jason, this is your thing. I just can't do it. I'm glad you told me about it, but I think I'll leave you and Bruce to this whole... Batman and Robin thing." You murmured, running towards the elevator, so you didn't have to look at the crest-fallen face of your older brother.

Only a week later you were standing in front of Jason's grave, tears spilling uncontrollably, with Bruce's arm over your shoulders. You pulled his arm off of your shoulder after a while and turned to walk away.
"Miss <y/n>?" Alfred's worried voice called.
"It's all your fault Bruce. If you hadn't taken him with you that day... I'd still have my older brother!" You quietly said, balling your hands into fists at your side.
"<y/n>... I'm so sorry. I-"
"Shut up." You replied, cutting his apology short and shooting a glare that could disintegrate 5 people in a second over your shoulder, and then walking away.
It took well over a year for you to warm up to Bruce again, and when you did you spent an hour just crying in his embrace, apologising for being so cold and letting out all your emotions that you'd been bottling up since the funeral. When you'd finally recovered, and all your tears were gone, you got up and brushed down your clothes.
"Train me so I can fight crime with you."
"W-What?" Bruce's smile was replaced by a look of shock. "I thought you said-"
"I know what I said, but... Jason wanted me to join you before you went on that mission. He was always so much happier since he started going out as Robin. I feel like if I started helping you I'd feel a little better..." You looked at him with so much hope, and smiled when his stare softened.
"You start right now, and I think I have the perfect alias for you..."
*****
You let out a sharp cry as you sat up, not quite knowing where you were. You looked around frantically, and locked eyes with Alfred, who had put his hands onto your shoulders.
"Miss <y/n>, please calm down." He soothed, slowly letting go as your breathing slowed back to normal and you realised that you were in your own room (Still in your costume). "I'll go get Master Bruce, he was worried sick about you."
Minutes later Bruce had entered, and before you knew it you were hugging him.
"I'm glad you're awake <y/n>. Do you remember what happened to you?" He asked, pulling away with his hands on your shoulders.
"Yeah... I took a short-cut so I could cut you guys off and finish the chase a little earlier... But I lowered my guard and that Red Hood guy got me from behind." You paused for a moment, placing a hand on your throbbing forehead and trying to remember that last important detail. "He said something to me... But I can't remember what."
"How about you get changed, and come down for something to eat?"
"But what about-"
"We can continue the search tomorrow. Right now you need to rest."
"There's something you're not telling me Bruce." You said, looking at him and seeing how he fiddled with his hands.
"I really did train you well didn't I?"
"Maybe I'm just a natural detective hm?"
"You're right. There is more... I think the Red Hood is-"
"Jason... My brother."