After Milly's boast, Char picked up Pico and ran out of her room without saying anything. She really didn't knew why she was so flustered, but she didn't wanted Milly to continue starting her down or else she would never be able to keep her cool and surprise the white haired girl.
The next thing that Char knew, she was in the bathroom, soaking away her nervousness while washing Pico's feathers with his favourite shampoo. Char wasn't really mad at Milly for blasting her because she really had it coming, but she wasn't feeling well about Pico having been afflicted too, "I'm sorry, Pico, I'll get you something delicious to ready later."
Char dressed in a set of clothes that were slightly more fancy than what one would usually wear on a pirate ship and went to the lower deck to put her devious plan into action. While she was preparing everything, Char didn't noticed the commotion upstairs but it was good in that because of it no one noticed that Char set the party supplies in place. When Melody called the whole crew to the upper deck Char and Pico had already set everything in place and planned to wait for the appropriate time to announce their surprise party.
During Melody's explanation about their cat-boy stowaway, Char still kept her wicked grin, but she actually found the boy to be kinda cute. Specially when he was curled up in fear like that, however she couldn't help but say,"Oh my, what a cute little kitty, perhaps now we can solve the ship's pest problem." while poking the boy's ears with a finger. After that she shot a suggestive glance at Milly.
When she was sure that the smallish girl was puffing her cheeks and balling her fists on anger, Char deftly presented her with a wrapped package that she was hiding the whole time, "Don't be like this Milly-san it's not good for you to be so furious on your birthday."
"...!?" It looked Milly was more surpised that Char called her by her name than by the sudden announcement of her birthday. She didn't picked the package right away however, fearing that it might be prank in revenge for the earlier flashbang.
"Come on, it's not a trap... do you think I'm that low?" Char forced the package in Milly's hands and urged her to open it. Just as the white haired girl unwrapped the package, Char couldn't her but notice how Song was looking wistfully at the (admittedly expensive) beautiful red wrapping paper as well as the bottled away disappointment when the paper was carried away by the wind.
Anyway, Char wasn't able to guess what was going through Milly's mind when she opened the package and found a beautiful summer dress, perfect for the kind of weather they usually faced in their travels, the dress also had a few extra pockets where Milly could stuff things (much likely bombs) so that it not only pretty but useful too. But when Milly took the dress out she found something that Char was sure that would surprise her, because Char guessed that it would be the very first bra that Milly ever had since a girl like her probably wouldn't go into a store an ask them for a custom one.
Char would leave the boasting for later since she had a party to begin, "Hey, everyone don't stand there like this, there's a party in the set in the cargo hold, hurry up and let's party! But, before that, where's Jon-sama?"
During the party...
Char sneaked away from it the rest of the crew and went to the same corner where Song was standing a little away of the others and, taking care for no one else to notice, gave her the sheets of colored paper, two yellow and one red, "I bought extra just in case messed the wrapping, if you want you can have this," Char said before turning her head to the side and scoffing, "...but don't think that I'm doing this because I like you or anything like this, it's just that I cant have my rival for Jon-sama's heart being so sad about some sheets of paper..."
After that Char turned to Milly and said, "Well, for the grand finale, my last present for tonight!" she then removed a cloth that was covering the bundle of fireworks that she had purchased earlier. "If this doesn't make this party go boom, noting more will."