What is this IC marked C55?
The only part I could find with a SOT23-6 package that I could find with a C55 marking is the OPA3355 from TI.



However, based on your extra information, this is probably not the right part.



After a bit more searching, I've managed to find a second option which does appear to make sense for the new pin-out. The SN74LVC1G3157, again from TI. This is an SPDT switch.

From the pin-out below, we can see that pin 2 and 5 are the GND and 3.3V supplies which match your table. We also have the select line which is pin 6 and that goes to the LDO_RST_N pin, which could make sense. Then pin 4 is the common terminal of the switch which is your output, and pins 1 and 3 are the other two switch terminals.



From page 18 of the datasheet, we can find the various marking codes, and C55 is one of them. That would make it an SC70-6 package, which looking at the picture again is entirely possible - in my first search I narrowed it down to SOT32-6, so hence missed this second part.

In terms of functionality, when the select pin is low, the A terminal is connected to B1 (pin 3) which in your case is just tied low. When the A terminal is high, B2 (pin 1) is connected, which is your input. 

This sort of makes sense as it means when the LDO_RST_N pin is low (i.e. reset asserted), the RX_OUT signal would be tied low. When LDO_RST_N is high (i.e. not in reset), the RX_OUT and RX_IN are connected. The easiest way to check would be to measure with a multimeter the resistance between pins 4 and 3 when reset is high, and again when reset is low and see if it is high-z and then low resistance respectively.