Pilot Parallel Pen as a Highlighter
Sometimes we get customers who tell us they use their Lamy or TWSBI 1.5 italic nibs as highlighters, filling them with a highlighter ink like Noodler's Firefly, or another light ink. 1.5 is a pretty broad nib for writing, and you can get some good calligraphic action out of it, but you can also consider using a Parallel Pen if you're ready to get serious about your highlighting.
The obvious advantage is that you can get a much broader nib that is usually available on standard fountain pens. The Parallel Pen is available in 1.5 mm, 2.4 mm, 3.8 mm, and 6.0 mm. This is one of the most popular calligraphy tools because of its ease of use (no messing around with dipping nibs and drips of ink), portability and reliability - it lays down an even, consistent line every time. Many professional and expert calligraphers use these pens daily. The highlighting samples were done with the green lid, 3.8 mm Parallel Pen.
The second advantage is that the Parallel Pen has slightly more controlled and drier flow than a regular fountain pen. It is nice and even and consistent. You typically get more shading and variation out of fountain pens; the parallel plates helps regulate the flow. You may still get some pooling at the end of your strokes, but it won't be as wet. This is particularly important if you're highlighting notes or text on lower quality paper at work or school. You can use highlighter ink like Noodler's Firefly, which is more of fluorescent colour, but you can also try lighter shades like Rohrer & Klingner's Helianthus (above) or J. Herbin's Bleu Azur (below). You can even try diluting some mid-range colours to get lighter shades. You can add distilled water and ink in a separate vial.
Probably its biggest disadvantage is that it's a long pen, definitely not a shirt pocket pen for on the go. I am assuming the design of the body has to do with its function as a calligraphy tool. If you have a pencil case or cup, or can leave it at your desk at work, it may not be a big deal. If you are someone who does a lot of highlighting, as a student or teacher or whatever it is you do, the Parallel Pen offers a good alternative to the marker highlighters that can dry out on you, growing fainter day by day. It comes with two proprietary Pilot cartridges, and you can use the accompanying "cleaning converter" as a converter to hold ink. I have heard that the converter may not work, and it is indeed a tight fit, but we use the converter in the Parallel Pen in the tester tray, and trust me when I say the pens in our tester tray are soldiers. And, as a bonus, you can practise your calligraphy with it too.