We have executed a cost-effective approach to produce a high-performance multifunctional human–machine interface (HMI) humidity sensor. The designed sensors were ecofriendly, flexible, and highly sensitive to variability in relative humidity (%RH) in the surroundings. In this study, we have introduced a humidity sensor by using carbon paper (as both a substrate and sensing material) and a silver (Ag) conductive ink pen. The carbon paper-based humidity sensor was developed by using a simple handwriting approach and the resulting devices exhibited excellent results including fast response/recovery times (12/24 s), a wide sensing range (30 to 85%), small hysteresis (1.1%), high stability (1 month), and repeatability. This high-performance humidity response could be attributed to the highly porous, hydrophilic, and permeable nature of carbon paper. Besides these features, the sensor offered high flexibility (100 bending cycles across different radii) and adaptability for uses like breath monitoring (through mouth and nose), proximity sensing (from multiple distances ranging from 1 to 10 cm), and depicting Morse code. This research work is a significant step forward in humidity sensing technology and the sustainable future of electronic devices by using a cost-effective, fast, and simple fabrication technique.