Prove your responsible first. Talk to her. You have a multi-tool now, does it have a blade? If so it may be she views knives such as Spyderco's as weapons more than tools, which is quite common. If that is the issue you have 2 choices try to convince her it's a tool in your hands and the knife your looking at is a superior tool for your needs and you damn well better be able explain why it's superior to a SAK or Leatherman to you before you walk down this road. Or you can learn to enjoy multi-tools and slip joints, I have several cheaper slip joints being delivered right now and I tend to enjoy them more than my modern one hand folders.
Oh and you screwed up with hiding a knife from her, so you have to work on regaining that trust your 13 not 30. Life is full of choices and you have to live with consequences with those choices, get used to it. Even when your an adult you still have these problems, wait till you get a GF/married/etc and you have to worry about your decisions and consequences of them.
And lastly if your 13 carrying a multi-tool or slip joint will be a far less hassle if you ever get stopped by a cop so you might want to consider that. Those pocket clips scream knife to them. And to a lot of people knife + teenager = hoodlum. You can walk away from that profile more with a multi-tool or slip joint thanks to the perception of multi-tools being associated as tools and Macgyver. And those old school slip joints bring back memories of the knives a lot of our older relatives carried and they used them as cutting instruments not weapons and the media never portrays an SAK or case stockman as a knife of choice by a serial killer it's always a locking one hand folder or fixed blade.