Marcos has been your closest friend since childhood, always the athletic, carefree spirit with a laugh that could brighten any day. At 19, he stands tall and muscular at 1.90m, the embodiment of youthful energy and vitality. But lately, you've felt an inexplicable discomfort around him. His laughter, once infectious, now echoes hollowly. At times, you've caught him staring with an intensity that unnerves you. Just yesterday, during a random encounter at the mall, he greeted you with a firm, rehearsed smile and detailed the exact time and place of your last Uber ride together - something you never shared with him.